sheepskin n.

ruminant mammal of the genus Ovis, as a domestic species, one of the animals most useful to humans, Old English sceap, scep, Northumbrian scap, from West Germanic *skæpan (source also of Old Saxon scap, Old Frisian skep, Middle Low German schap, Middle Dutch scaep, Dutch schaap, Old High German scaf, German Schaf), a word of unknown origin. Not found in Scandinavian (Danish has faar for "sheep") or Gothic (which uses lamb), and with no known cognates outside Germanic. The more usual Indo-European word for the animal is represented in English by ewe.

The plural was leveled with the singular in Old English, but Old Northumbrian had a plural scipo. It has been used from Old English times as a type of timidity and figuratively of those under the guidance of God. The meaning "stupid, timid person" is attested from 1540s.

The image of the wolf in sheep's clothing was in Old English (from Matthew vii.15); that of separating the sheep from the goats is from Matthew xxv.33; the phrase itself by 1570s. To count sheep in a bid to induce sleep is recorded from 1854 but seems not to have been commonly written about until 1870s. It might simply be a type of a tedious activity, but an account of shepherd life from Australia from 1849 ["Sidney's Emigrant's Journal"] describes the night-shepherd ("hut-keeper") taking a count of the sheep regularly at the end of his shift to protect against being answerable for any animals later lost or killed.

Sheep's eyes "loving looks" is attested from 1520s (compare West Frisian skiepseach, Dutch schaapsoog, German Schafsauge). A sheep-biter was "an ill-trained mongrel, a dog that worries sheep" (1540s) and had extended senses: a mutton-monger" (1590s); and "a whore-monger" (1610s, i.e. one who "chases mutton"); hence Shakespeare's sheep-biting "thieving, sneaky." An old London chronicle c. 1450 has went to sheep-wash for "were slain."

c. 1200, "animal hide" (usually dressed and tanned), from Old Norse skinn "animal hide, fur," from Proto-Germanic *skinth- (source also of rare Old English scinn, Old High German scinten, German schinden "to flay, skin;" German dialectal schind "skin of a fruit," Flemish schinde "bark"), from PIE *sken- "to peel off, flay" (source also of Breton scant "scale of a fish," Irish scainim "I tear, I burst"), extended form of root *sek- "to cut."

The usual Anglo-Saxon word is hide (n.1). The meaning "epidermis of a living animal or person" is attested from early 14c.; extended to fruits, vegetables, etc. late 14c. Jazz slang sense of "drum" is from 1927. As short for skinhead from 1970. As an adjective, it formerly had a slang sense of "cheating" (1868, compare the verb); that of "pornographic" is attested from 1968. Skin deep "superficial, not deeper than the thickness of the skin" (also literally, of wounds, etc.) is attested by 1610s:

All the carnall beauty of my wife, Is but skin-deep.

[Sir Thomas Overbury, "A Wife," 1613; the poem was a main motive for his murder]

The skin of one's teeth as the narrowest of margins is attested from 1550s in the Geneva Bible, a literal translation of the Hebrew text in Job xix.20. To get under (someone's) skin "annoy" is from 1896. Skin graft is from 1871. Skin merchant "recruiting officer" is from 1792 (the older sense is "dealer in hides"). Skin and bone as a description of emaciation or extreme leanness is in Middle English:

Ful of fleissche Y was to fele, Now ... Me is lefte But skyn & boon. [hymn, c. 1430]