Green’s Dictionary of Slang

(a) (also skin) a college diploma, received on graduation [such diplomas are trad. made of sheepskin-based parchment].

1804		D. Webster Private Correspondence I 173: Feeling some anxiety about your ‘sheep-skin,’ I wrote to Merrill [DA].
1833		Albany Microscope (NY) 5 Jan. n.p.: Those who are called the best of men; / Those who in ‘sheep-skin’ covering cheat and lie.
[1834		N. Ames ‘Morton’ An Old Sailor’s Yarns 183: Before the signature [...] was dry on his sheep-skin diploma].
1843		R. Carlton New Purchase I 174: let not uncle Tommy’s pulpit preparation be despised; even ‘high larned sheepskins,’ it is said, do sometimes lay both the living and the dead. [Ibid.] 203: This apostul of ourn what spoke the text, never rubbed his back agin a collige, nor toted about no sheepskins.
1849		in N.E. Eliason Tarheel Talk (1956) 125: You must study hard to get your skin.
1854		‘A Little More Cider’ in C. Elliot Songs of Yale (1870) 31: In spite of scrapes and flunks, I’ll have a sheep-skin too.
1862		Yale Literary Mag. xxvi 147: Some of us [have] no aspirations beyond an easy course, and a sheepskin after four years.
1871		L.H. Bagg Four Years at Yale 47: Sheepskin, the college diploma, or A.B. degree.
1888		H.C. Tanner Lobby 370: When asked by the late Chief Justice Church where his ‘sheep-skin’ was, he replied that he never had but one sheepskin in his life, and that his father gave him when one of his sheep died [DA].
1895		J.S. Wood Yale Yarns 296: Once they had received their ‘sheep-skins,’ they rushed out on the green in boisterous squads.
1902		Conservative (Nebraska City, NE) 13 Mar. 5/3: Sons of sturdy fathers [...] waving their sheepskin diplomas ahead of them wherever they go.
1909		L.W. Payne Jr ‘Word-List From East Alabama’ in DN III:v 368: sheepskin, n. Diploma.
1926		Wood & Goddard Dict. Amer. Sl.
1939		W. Winchell On Broadway 10 Jan. [synd. col.] Charles R. Hires [...] and Virginia Lynch, the Drexel Hill beauty, are to wed as soon as he gets his sheepskin.
1949		St Paul Pioneer Press 19 June 1/4: Under the GI Bill of Rights, the dogged, over-age collegian [...] bought his sheepskin at the price of uncounted mental, physical and spiritual outlay [DA].
1957		D. Burley N.Y. Amsterdam News 24 Aug. 13: Sheepskins testifying they can read Greek, Latin, ‘oo-bop-she-skoolymop’ and Sanskrit.
1963		F. Kohner Affairs of Gidget 64: I gotta hold out till the end of the season, and till I get the ol’ sheepskin.
1979		S. King Dead Zone (1980) 23: A grunt is a student who gives a shit about nothing except his sheepskin.
1987		H.L. Horowitz Campus Life 183: The large majority of students devoted themselves to studying and getting their sheepskins.
1989		J. Morton Lowspeak 127: Sheepskin – a certificate hung in a lawyer’s office showing his qualifications.
1995		J. Stahl Permanent Midnight 37: It’s not like that sheepskin from Columbia was doing me any good.
2011		Chicago Trib. 12 Oct. 1/1: Recent college graduates may feel like their sheepskin isn’t worth much these days.

(b) a document.

1820		J. Poole Paul Pry II i: I made him sign a bond [...] you see I always preserve sheep-skin, before anything else.
1853		Dickens Bleak House (1991) 443: Some wise [...] conveyancer yet toils for the entanglement of real estate in the meshes of sheep-skin.
1853		G.J. Whyte-Melville Digby Grand (1890) 9: Fat Mr Sheepskin, the lawyer.
1884		Henley & Stevenson Admiral Guinea I vii: Hey! what’s this?—a book—sheepskin.
1903		A. Binstead Pitcher in Paradise 77: It was at no suggestion of mine that I signed so many sheepskins.

(c) (US prison) a pardon or discharge certificate.

1931		Irwin Amer. Tramp and Und. Sl. 168: Sheepskin.– A pardon from the Governor or President, such a document formerly being written on parchment.
1949		Monteleone Criminal Sl. (rev. edn).
1962		Ragen & Finston World’s Toughest Prison 817: sheepskin – A pardon from the Governor or President.