thievish (adj.)

Old English þeofian "to thieve, steal," from þeof (see thief). Rare in Old English, rarer in Middle English, not common until 17c.; perhaps the modern word is a late 16c. re-formation. Thieving (adj.) first attested 1520s.

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]

adjectival word-forming element, Old English -isc "of the nativity or country of," in later use "of the nature or character of," from Proto-Germanic suffix *-iska- (cognates: Old Saxon -isk, Old Frisian -sk, Old Norse -iskr, Swedish and Danish -sk, Dutch -sch, Old High German -isc, German -isch, Gothic -isks), cognate with Greek diminutive suffix -iskos. In its oldest forms with altered stem vowel (French, Welsh). The Germanic suffix was borrowed into Italian and Spanish (-esco) and French (-esque). Colloquially attached to hours to denote approximation, 1916.

The -ish in verbs (abolish, establish, finish, punish, etc.) is a mere terminal relic from the Old French present participle.