Origin and history of tallish

"having a relatively great stature, high in proportion to breadth," 1520s, originally of persons; by 1540s of things, probably from otherwise obsolete Middle English tal "handsome, good-looking;" also "valiant," from Old English getæl "prompt, active," from Germanic *(ge)-tala- (source also of Old High German gi-zal "quick," Gothic un-tals "indocile").

The sense of "being of more than average height (and slim in proportion to height)" probably evolved out of earlier meanings "brave, valiant, seemly, proper" (c. 1400), "attractive, handsome" (late 14c.), also "large, big" (mid-14c.), as sometimes in Modern English, colloquially.

The sense evolution is "remarkable," says OED (1989), but it notes that adjectives applied to persons can wander far in meaning (such as pretty, buxom, German klein "small, little," which in Middle High German meant the same as its English cognate clean (adj.)).

The meaning "having a (defined) height," whether lofty or not is from 1580s. The meaning "exaggerated" (as in tall tale) is American English colloquial attested by 1846. Phrase tall, dark, and handsome is recorded by 1841. Related: Tallness.

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.