[Old English -isc, of Germanic origin; related to Old Norse -iskr, German and Dutch -isch, also to Greek -iskos (a suffix forming diminutive nouns).]
This suffix forms adjectives from nouns and from other adjectives.
One set from nouns is of adjectives for a member of a nation (British, Cornish, Irish, Netherlandish, Polish, Swedish, Turkish), occasionally of a religious or other group, as with Jewish. Rarely, the ending has been reduced to -sh, as in Welsh.
A second set from nouns is of adjectives that indicate its qualities or characteristics (boyish, folkish, gypsyish). Many are formed from proper names, often for a single use: Ayckbournish, Hockneyish, Thurberish. Though these examples are neutral in tone, the great majority are derogatory: babyish, Bunterish, bookish, boorish, childish, Eeyorish, foolish, freakish, Micawberish, priggish, selfish, sluttish.
Examples formed from other adjectives suggest some quality that is roughly or somewhat like that of the adjective (coldish, dullish, loudish, moreish, oldish, sweetish, tallish, weakish). In particular, the ending can suggest an approximate age (fiftyish) or time of day (sevenish, latish).