mid-15c., "exalted, glorious, great in actions or deeds," from Old French magnificent, a back-formation from Latin magnificentior, comparative of magnificus "great, elevated, noble, distinguished," literally "doing great deeds," from magnus "great" (from PIE root *meg- "great") + combining form of facere "to make" (from PIE root *dhe- "to set, put"). Meaning "characterized by grandeur or stateliness; living in splendor" is from 1520s. As an exclamation expressing enthusiastic admiration, by 1704.
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.