Where does the term "shiv" (a makeshift dagger) actually come from?

Solution 1:

Shiv (preferred spelling in US slang) is from chiv or chive in thieves' cant (from British slang). Chiv is from chivomengro (meaning knife in Gypsy or Romani language) according to several sources. Most cant terms are said to be from Anglo-Romani language which has roots in Sanskrit. The word might be related to the Gypsy word churi (meaning knife) and ultimately Sanskrit word छुरी (churī) (meaning knife, dagger).

OED mentions that chiv(e) (n) is from thieve's cant and has the earliest usage from 1673:

R. Head Canting Acad. 12      He takes his Chive and cuts us down.

Listed below are the sources mentioning chivomengro:

From Modern Language Notes (by Johns Hopkins Press):

Another cant term which may have come from the Romani is chive 'knife'. This word first noted in 1673 (N.E.D.) is still in use at the present time. The preferred spelling is chive , but it is often given as chivvy , chiv, shiv , or shive. The pronunciation is either 'shiv' (rhyming with 'to live') or 'shivvy'. There is a Gypsy word chivomengro 'knife' whose stem 'chiv' suggests itself as the origin of the underworld chive.

From Wise Young Fool (by Sean Beaudoin):

A shiv (from the Romani word chivomengro, or "knife") is a slang term for any sharp or pointed implement used as a weapon. Inmates in prisons around the world make shivs.

From Journal of the Gypsy Lore Society (by The Society):

Some sources say that it is from Gypsy chive meaning 'to stab':

From "A Dictionary of Slang, Jargon & Cant: Embracing English, American, and Anglo-Indian Slang, Pidgin English, Tinker's Jargon and Other Irregular Phraseology" by Albert Barrère, Charles Godfrey Leland:

From "A Classical Dictionary of the Vulgar Tongue" by Francis Grose:

Solution 2:

Shiv is of Romany origin. It is included by George Borrow in his Romano Lavo-Lil Word-Book of the Romany or English Gypsy Language (1874):

• chiv, chiva, chuva: verbum activum to cast, fling, throw, place, put.

Eric Partridge in his Dictionary of Slang and Unconventional English (3rd edition, 1949) has the following about chive or chiv which are variants of shiv:

• chive or chiv - of Romany origin - 17th-20th century cant for a knife, a file, a saw; Romany and cant for to stab, to cut or saw (through), to ‘knife’: mid 18th-20th century (Grose, 1st edition)

Grose refers to A Classical Dictionary of the Vulgar Tongue first published in 1785 where chive or chife is defined as a knife, file or saw. Partridge edited Grose's dictionary and added comments to it. Here is Partridge's addition to chive:

• In Romany chive has the specific sense to stab, from chiv, chiva (chive), chuva, to cast, fling, throw, also to place, put. G.B.

G.B. stands here for George Borrow and brings us back to the Romano Lavo-Lil.

Yet more information is found in Partridge's Dictionary of the Underworld British & American (1949):

• Shiv, noun, is a variant of shive, than which, indeed, it is much more common. In e.g. Ersine [Noel Ersine, Underworld and Prison Slang, 1933]
• Shive A knife, late 19th-20th century. Donald Lowrie, who was at San Quentin in 1901-11, makes an old hand say, in reference to the 1890's, ‘I always carried a shive m'self. Everybody carried a shive. Y'r had t' carry one.’ (My life in Prison, 1912)

It is interesting to note that in numerous languages one of the slang words for knife has also been borrowed from Romany but directly from ćhuri meaning knife in that language. It appears for example in French as chourin, which is now obsolete, or surin, which is still used. Wartburg in his Französisches Etymologisches Wörterbuch has the following remark about it:

• chourin ist aus dem zigeunerischen churí entlehnt, so auch im argot von Barcelona xurí, im calo Spaniens churí.
• chourin is borrowed from Romany, as in Barcelona slang xurí and in Spanish caló churí.

Solution 3:

Green's Dictionary of Slang appears to support the Romani origin with the earliest usage of chiv from the second half of the 17th century and shiv from the late 19th century:

chiv n.¹

also chev, chevy, chib, chieve, chiff, chive, chivie, chivvy, chivy, skiv

[Rom. chiv, chive, a knife]

1. (UK Und.) a knife or razor.

   • 1674 [UK] ‘Of the Budge’ HEAD Canting Academy (1674) 12: For when he hath nubbed us, / And our friends tips him no cole, / He takes his Chive and cuts us down / And tips us into the hole.

shiv n.

also schieve, sheive, shieve, shive, shivy

[CHIV n.¹]

• 1897 [US] J. LONDON ‘The Road’ in HENDRICKS & SHEPHERD Jack London Reports (1970) 311–21: Their argot is peculiar study. [...] shiv, knife.

The Concise New Partridge Dictionary of Slang and Unconventional English suggests a different origin of shiv:

Probably from “shive” (to slice bread), 1570.