late 14c., "grasped by the understanding" (rather than by the senses), from Old French intellectuel (13c.) and directly from Latin intellectualis "relating to the understanding," from intellectus "discernment, understanding," noun use of past participle of intelligere "to understand, discern" (see intelligence).
Sense of "characterized by a high degree of intellect" is from 1819. Meaning "appealing to or engaging the mental powers" is from 1834. Intellectual property "products of the intellect" is attested from 1845. Adjective formations in the sense "of or pertaining to the intellect" included intellective (early 15c.), intellectile (1670s).
common adverbial suffix, forming from adjectives adverbs signifying "in a manner denoted by" the adjective, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cognates: Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). Cognate with lich, and identical with like (adj.).
Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.