A suffix used to from adjectives from nouns and from adjectives. It denotes relation, resemblance, similarity, and sometimes has a diminutive force; as, selfish, boyish, brutish; whitish, somewhat white. [1913 Webster]

   A verb ending, originally appearing in certain verbs of French origin; as, abolish, cherish, finish, furnish, garnish, impoverish. [1913 Webster]

-ish, suffix forming adjectives:
1 from nouns, meaning: a having the qualities or characteristics of (boyish). b of the nationality of (Danish).
2 from adjectives, meaning 'somewhat' (thickish).
3 colloq. denoting an approximate age or time of day (fortyish; six-thirtyish).

-ish, suffix (also -ise) forming verbs (vanish; advertise).

Etymology:
from or after F -iss- (in extended stems of verbs in -ir) f. L -isc- incept. suffix