Where does the word “wankers” come from?
I would say it has onomatopeic roots.
Firstly, this isn't the only word for masturbation to have echoic roots: fap is echoic of the noise of masturbation.
Looking at the entry in the OED, some of the uses spell the word whank, and in fact one example mentions that it might be echoic:
1951 E. Partridge Dict. Slang (ed. 4) 1220/1 Whank, (male) self-abuse: low: from ca. 1870. Perhaps echoic.
Looking through Google books, it shows that whank is used as echoic of bird calls:
For example
"Whank, whank, whank!" the little bird calls.
Also in the wank form:
"Wank, wank, wank", it trumpets, in a musical, nasal tone
Given that the OED's earliest citation (1948, British RAF) refers to a bed, I would think that wank is echoic of the sound of bed boards or mattress springs.
Recent slang dictionaries on the origin of 'wank' and 'wanker'
I note at the outset that every recent slang dictionary ultimately concedes that "origin unknown [or obscure]" remains the final word on these terms. Nevertheless, some dictionaries are more inclined than others to entertain speculation.
Starting with the most cautious treatment, we have John Ayto & John Simpson, The Oxford Dictionary of Modern Slang (1992):
wank noun 1 An act of (male) masturbation. 1948–. [Example from 1977 omitted.] 2 = WANKER noun 2; also, an objectionable thing. 1970–. [Example from 1977 omitted.] 3 To masturbate; often followed by off. 1950–. [Examples from 1977 and 1984 omitted.] {Origin unknown}
wanker noun 1 A male masturbator. 1950–. [Example from 1971 omitted.] 2 An objectionable or contemptible person, esp. male; spec an incompetent, pretentious, or ostentatious person. 1972–. [Example from 1981 omitted.] {From WANK verb + -er.}
Tony Thorne, The Dictionary of Contemporary Slang (1990) is more speculative:
wank vb British 1 to masturbate. This very widespread vulgarism (with some recent exceptions still taboo in the printed and broadcast media) is, perhaps surprisingly, of obscure origin. It seems to have entered the spoken language in the late 19th century, significantly at a time when the word whang was emerging as a vulgar term for penis. Wank (earlier spelled 'whank') is probably derived from the same source; 'whang' as a dialect word first meaning hit, beat or slap. Wank may be simply a variant pronunciation, or a development of the earlier word influenced by 'whack' and 'yank'. Since the 1960s the word has been used of and by women as well as men. 2 to behave in an ostentatious, self-indulgent and/or futil manner. A usage deriving from the interpretation of masturbation as purposeless and/or offensive.
wanker n British 1 a masturbator. For the probable etymology of the word see wank. 2 an inconsequential, feeble, self-indulgent or otherwise offensive person. The term of abuse or disapproval (most frequently applied to males) has been in use since the early 20th century, but became extremely common in the 1970s. In the USA the word is known, but its force as a taboo term in Britain is often underestimated by American speakers.
Robert Chapman & Barbara Kipfer, Dictionary of American Slang, third edition (1995) concedes the "origin unknown" point, but then permits itself a "perhaps from" explanation:
wank 1 v (also wank off) late 1940s British To fondle one's own penis; =JACK OFF, BEAT one's MEAT ... 3 n by 1970 A contemptible person; =GEEK, JERK, DORK: [example omitted] 4 v 1990s Canadian students To have fun; =PARTY, HAVE A BALL
wanker 1 n chiefly British fr late 1940s A masturbator, either literally or figuratively; =JERK-OFF: [examples omitted] 2 n 1990s Canadian students A fun-loving person; =PARTY ANIMAL 3 n 1990s students The penis {origin unknown; perhaps fr British dialect wank, "a violent blow," and semantically analogous with beat one's meat, whack off, pound one's peenie, etc}
The wanky world in 1937
Perhaps the oddest thing about the way the three slang dictionaries cited above handle wank and wanker is the discrepancy in their date of origin for wank and wanker in a sexual sense. Ayto & Simpson gives 1948 for wank and 1950 for wanker, Chapman & Kipfer has late 1940s for both, and Thorne has "late 19th century" for wank and "early 20th century" for wanker. Why the multi-decade disagreement?
To make sense of this issue, I turned to the first edition of Eric Partridge, A Dictionary of Slang and Unconventional English, a massive (975-page) dictionary of (mostly UK) slang published in 1937 and conveniently retained in its 1937 form in the fifth edition (1961), which compiles all of Partridge's post-1937 additions as a supplement that runs to almost 400 pages. Partridge's approach is encyclopedic and he's not at all shy about reporting off-color usage, so the fact that his 1937 dictionary has no mention of wank and wanker in their modern senses is a serious mark against Thorne's dating.
From an etymological perspective, however, Partridge's 1937 dictionary is surprisingly rich with wank-related possibilities. Here's a look at some of the entries in that dictionary.
wank. See wonk.
wanker. A bloater : Felsted School : 1892, The Felstedian, Oct.; ibid., June, 1897, 'He sniffs, 'eugh, wankers again.' Ex stinker (via stwanker).
wanky, wonky. Spurious, inferior, wrong, damaged or injured : printers' (— 1904), by 1914, gen. 'A wanky tanner = a snide ["counterfeit"] sixpence,' F[armer] & H[enley, Slang & Its Analogues (1904)]. Prob. ex dial. wankle, unsteady, precarious ; delicate in health ; sickly. In East Anglian doil., wanky is 'feeble' : [Joseph Wright,] E[nglish] D[ialect] D[itionary].—2. Hence, nervous, jumpy : Air Force from 1915. F[raser] & Gibbons, [Soldier & Sailor Words and Phrases (1925)]
...
whang. A 'whanging' sound or blow : dial. (— 1824) and, from ca. 1860, coll. Ex:
whang, v.t. To strike heavily and resoundingly : coll.: C. 19–20. Ex dial. (C. 17–20). Echoic.—2. V.i. (of e.g., a drum), to sound (as) under a blow: coll. : 1875, Kingslake (O.E.D.).
{whangam, whangdoodle, An imaginary animal : rather nonce-words than coll.}
...
wonk. A useless seaman ; a very inexperienced naval cadet : naval : from ca. 1917. Bowen[, Sea Slang (1929)] Ex wonky.
wonk, all of a. Upset, very nervous : 1918 (O.E.D. Sup.) ; ob. Ex:
wonky. See wanky.
Nothing in Partridge's lineup seems on the verge of metamorphosing into wank or wanker in the modern sense, but there are certainly some intriguing suspects to consider here—in particular, the resounding and struck senses of whang and the various senses of wanky/wonky.
Wanking revisited in a post-Depression world
As the answer by Review queue fodder (above) suggests, once wank and wanker caught on in the UK, Partridge went back and, for the fourth edition (1951) of his dictionary, found multiple relevant antecedents that he had theretofore overlooked, with usage going back to the nineteenth century—including whank ("to masturbate: low: late C. 19–20"), whank-pit ("A bed: R.A.F.: since ca. 1920"), whanker ("a masturbator: low" late C. 19–20), whanker's colic "An undiagnosed visceral pain: R.A.F/: since early 1920's"), whanker's doom ("Debility: R.A.F.: since ca. 1925"), whanking-pit ("the Army's form of whank-pit: since early 1920'"), and whanking-spanner ("An imaginary tool like a 'sky-hook'" and "The hand: low: since ca. 1920").
A Google Books search for whank and whanker doesn't find any instances of either term from the early twentieth century; instead, it finds a very old occurrence of both terms in C. Clough Robinson, A Glossary of Words Pertaining to the Dialect of Mid-Yorkshire (1876):
Whank [waangk'], a large portion ; gen. 'A whanking lump' [U waang'kin luomp']. 'That 's a whank big enough' [Dhaate' u waangk' big' uni'h'f]. 'A whanker' [U waang'kur].
Ultimately, the extent to which you're inclined to find the origins of present-day wank and wanker in the 1940s (as Ayto & Simpson does) rather than in the late 1800s (as Thorne does) may reflect your view of Partridge's belated (and not well documented in his own book) discovery in the 1940s of whank and its allies in a line of use stretching back over a period of four to six decades.
The German verb "wanken" means "to go back and forth."
That seems to describe the British word "wank," which means "to go back and forth" below one's belt.