c. 1200, "resembling a sheep" in some perceived characteristic, from sheep + -ish. The sense of "bashful, over-modest, awkward among strangers" first is recorded 1690s. Related: Sheepishly; sheepishness. Old English had sceaplic "of a sheep, 'sheep-ly.'"
sheela-na-gig
sheen
sheeney
sheep
sheep-dog
sheepish
sheep-shank
sheepskin
sheer
sheet
Sheetrock