"fashionable, stylish," often with a hint of contempt, 1650s, from mode (n.2) + -ish. "Very common in 17-18 c.; now somewhat arch[aic]." [OED]. Related: Modishly; modishness.
Entries linking to modish
mode n.2
"current fashion, prevailing style," 1640s, from French mode "manner, fashion, style" (15c.), a specialized use of the French word that also yielded mode (n.1).
-ish
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.