"of or pertaining to Cornwall," the county in the far southwest of England, late 14c. (as a surname), from first element of Cornwall (q.v.) + -ish. Earlier was Cornwalsche, Cornwalish (c. 1200). As a noun, the name of the ancient Celtic language of Cornwall, extinct as a spoken language since late 18c., from 1540s. Related: Cornishman.
Ending changed mid-14c. to -ish under influence of ravish, anguish, etc. It also once had an intransitive sense and was so used by Shakespeare and Milton. Related: Famished; famishing.
It has naturally been assoc. with PET sb.1, as being a characteristic habit of a "pet" or indulged and spoiled child; but the connexion of sense is not very clear or simple .... [OED]
Related: Pettishly; pettishness.