Green’s Dictionary of Slang

jemmy n.1

also jimmy
[ety. unknown]

1. a fool.

[UK]J. Taylor Laugh and Be Fat 5: I thinke it more than foolish pittie, / So great a iemme as you should grace the citie.

2. a wig.

[UK]G.A. Stevens in Adventures of a Speculist (1788) II 93: The Jehu’s Jemmy, or ‘White and All-White,’ in little curls, like a fine fleece on a lamb’s back, we should say something upon, were it not for fear of offending some Gentleman of Great Riches who love to look like coachmen.

3. a sheep’s head [Bee (1823) suggests an actual butcher Jemmy Lincomb, who lived near Scotland Yard].

[UK]Poor Law/ Overseers Accounts, Great Barr, Staffs. n.p.: A bot o’ sheep’s jimmy with baked taerters and taernips.
[UK]Vaux Memoirs 156: BLOODY-JEMMY, a sheep's head.
[UK]‘An Amateur’ Real Life in London I 385: D—n me if I would give a pair of crazy crabshells without vamp or whelt for the whole boiling of ’em — there is not one of ’em worth a bloody jemmy.
[UK]Dickens Oliver Twist (1966) 201: She presently returned with a pot of porter and a dish of sheep’s heads: which gave occasion to several pleasant witticisms on the part of Mr Sikes, founded upon the singular coincidence of ‘Jemmies’ being a cant name, common to them, and also to an ingenious implement much used in his profession.
[UK]W.T. Moncrieff Scamps of London II iii: I shall stand a Jemmy and sauce at Mother Potter’s in the Cut.
[UK]W. Phillips Wild Tribes of London 96: Baked-potato shops [...] combine the attractions of the ‘jemmy’ or sheep’s head business with the other.
[UK](con. 1840s–50s) H. Mayhew London Labour and London Poor II 42/1: They [...] had a ‘prime hot Jemmy a-piece,’ with a drop of good beer. A ‘Jemmy’ is a baked sheep’s head.
[UK]Hotten Sl. Dict.
[UK]G.A. Sala in Living London (1883) Sept. 422: Sheep’s head is a shamefully neglected viand [...] I presume because the commonalty call it a ‘jemmy’.
[UK]Sporting Times 29 Mar. 2/1: The sheep’s-jimmies (or ‘mountain peckers’) were superb, whilst the ’74 Castle U.P. was a poem.
[UK]G. Squiers Aerbut Paerks, of Baernegum 46: A bit o’ sheep’s jimmy with baaked taerters.

4. a large human head.

[UK]G. Kent Modern Flash Dict.
[US]Sun. Mercury (N.Y.) 7 Feb. 2/2: When in the street, [...] his head is encircled by an ordinary hat, one of the Champlins, which he wears in such a way as would lead the uninitiated to suppose that he intended it for the protection of his sconce or jemmy as it is called in the classics, rather than for elegance.
[UK]C. Hindley Vocab. and Gloss. in True Hist. of Tom and Jerry 181: Jemmy. A head.