"pertaining to or native to Flanders," early 14c., flemmysshe, probably from Old Frisian Flemische, or a native formation from Fleming + -ish.
Entries linking to flemish
Fleming n.
from Old English Flæming "native or inhabitant of Flanders," from Old Dutch Vlaemingh, Old Frisian Fleming, both from Proto-Germanic *Flam- (see Flanders). The Germanic name was borrowed in Medieval Latin as Flamingus, hence Spanish Flamenco, Provençal Flamenc, etc. French has flandrin "a lanky lad" (15c.), originally a nickname of a Fleming, thence "any tall and meagre man," as they were thought to be [Kitchin].
-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.