Origin and history of dimmish

Old English dimm "dark, gloomy, obscure; not clearly seen, indistinct," from Proto-Germanic *dimbaz (source also of Old Norse dimmr, Old Frisian dim, Old High German timber "dark, black, somber"). Not known outside Germanic.

Of eyes, "not seeing clearly," early 13c. Of sound from early 14c.; of light, "not bright, faintly luminous," from early 14c. Modern slang sense of "dull of apprehension, stupid" is from 1892; the sense of "dull-witted" also was in Middle English (mid-13c.). Related: Dimly; dimness.

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.