thieve (v.)

Old English þeof "thief, robber," from Proto-Germanic *theuba- (source also of Old Frisian thiaf, Old Saxon thiof, Middle Dutch and Dutch dief, Old High German diob, German dieb, Old Norse þiofr, Gothic þiufs), a word of uncertain origin.

A thief takes other people's property without their knowledge ; a robber takes it openly, whether or not resistance is offered : in a looser sense, thief is often applied to one who takes a small amount, and robber to one who takes a large amount. [Century Dictionary, 1895]

1560s, from thieve + -ery. An Old English word for it was þeofend.

mid-15c., "of or pertaining to thieves," from thieve or thief + -ish. Meaning "inclined to steal" is from 1530s. Wyclif and Chaucer used thiefly (late 14c.). Related: Thievishly; thievishness.