"pre-eminently, above all, in the first place," mid-14c., from chief (adj.) + -ly (2). The adjectival meaning "pertaining to a chief" is by 1870 (from chief + -ly (1)).
Entries linking to chiefly
chief adj.
c. 1300, "highest in rank or power; most important or prominent; supreme, best, placed above the rest," from Old French chief "chief, principal, first" (10c., Modern French chef), from Vulgar Latin *capum (source also of Spanish and Portuguese cabo, Italian capo, Provençal cap), from Latin caput "head," also "leader, guide, chief person; summit; capital city" (from PIE root *kaput- "head").
-ly 2
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.
-ly 1
suffix forming adjectives from nouns and meaning "having qualities of, of the form or nature of" (manly, lordly), "appropriate to, fitting, suited to" (bodily, earthly, daily); irregularly descended from Old English -lic, from Proto-Germanic *-liko- (Old Frisian -lik, Dutch -lijk, Old High German -lih, German -lich, Old Norse -ligr), related to *likom- "appearance, form" (Old English lich "corpse, body;" see lich, which is a cognate; see also like (adj.), with which it is identical).