pettish adj.

"fit of peevishness, offense or ill-humor at feeling slighted," 1580s, in phrase take the pet "take offense." Perhaps from pet (n.1) on a similar notion to that in American English that gets my goat (for which see goat), but the underlying notion is obscure, and the form of the original expression makes this doubtful. Perhaps from the notion of "characteristic of an indulged or spoiled child," but OED notes that this word seems to have been originally a southern English term, while pet (n.1) was northern and Scottish. Perhaps influenced by unrelated petulant.

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.