Why are promiscuous women known as "slappers"?

Women who aren't interested in much more than sex are referred to as "slappers" in British English.

British informal, derogatory a promiscuous or vulgar woman.

Why is this? I can't find any solid etymology information.

OED has this as a draft addition from 2002 (not everything is reproduced here):

Brit. slang (derogatory). A promiscuous woman. Freq. in old slapper.
See quot. 1990 for a postulated connection with Yiddish schlepper ‘unkempt, scruffy person; gossipy, dowdy woman’; however there is some gap in sense. Cf. also quot. 1854¹ at sense 1.

1990 T. Thorne Bloomsbury Dict. Contemp. Slang 468/1 Slapper in British, a prostitute or slut. This working class term from East London and Essex is probably a corruption of shlepper or schlepper, a word of Yiddish origin, one of whose meanings is a slovenly or immoral woman.

The 1854 quotes in sense 1 are:

1. dial. A large thing or object; a big, strapping, or overgrown person.

1854 A. E. Baker Gloss. Northamptonshire Words, Slapper,..applied to persons and things, but most frequently to over-grown females.
1854 A. E. Baker Gloss. Northamptonshire Words, ‘She's a slapper.’

Both etymologies are possible. I guess that “over-grown females” might slap parts of the anatomy against other parts more than smaller women.

From there, it’s but a small step to an assumption of promiscuity and a corresponding shift in meaning. Unfortunately, slang is anything but politically correct.

There are several possible explanations for the British slang term slapper, which some suggest is the equivalent of the American term, tramp — a woman who has sex with many different men.

1) It could derive from the theatrical expression:

to slap on the greasepaint

The Theatre Dictionary: British and American Terms in Drama, Opera, and Ballet by Wilfred Granville, 1970, says

There is evidence to suggest that the noun slap meaning theatrical make-up, existed pre 1860, but was already losing favour in the 19th century.

Well, that prediction proved to be untrue as shown by ODC

Slap 2 [mass noun] informal make-up, especially when applied thickly or carelessly: I put a bit of slap on my face and we were ready to go

Now, anyone who is British-born will be familiar with the notion that slappers are supposedly ill-famed for troweling on their make-up, or war paint if you prefer. Hence, a promiscuous woman is identifiable as someone who slaps on her make-up heavily.

2) The idiomatic expression "a bit of slap and tickle" meaning amorous frolicking, with kissing, petting etc. is said to have existed since 1910 in the Shorter Slang Dictionary by Paul Beale, and Eric Partridge.

The word slap mimics the sound of flesh on flesh, or the sound of heavy breasts slapping together. It could also suggest that a slapper is a woman who is up for some slap and tickle. In other words a willing sexual partner.

3) Finally, in Curious English Words and Phrases: The Truth behind the expressions we use its author, Max Cryer, suggests that slapper could stem from the Irish sliobaire meaning dirty. In trying to find a direct translation of sliobaire I was largely unsuccessful, although I did find this reference, the Dictionarium scoto-celticum:

Slìobadh, -aidh, s. m. et pres. part. v. Sliob. 1.
Stroking, act of stroking, or rubbing gently with the hand : palpandi demulcendi actus. C. S. 2. Licking, act of licking : lanibendi actus. C. S. 3. Polishing, act of polishing : laevigandi actus. C. S.
Sliobair, -e, -ean, s. m. A clumsy, or awkward fel- low : inconditus, vel inhabilis quis. C. S.

Not being Irish I am unable to ascertain whether these words have any phonetic similarities with "slap" or "slapper".