early 15c., "done with careful consideration," from Latin deliberatus "resolved upon, determined," past participle of deliberare "consider carefully, consult," literally "weigh well," from de, here probably "entirely" (see de-) + -liberare, altered (probably by influence of liberare "to free, liberate") from librare "to balance, make level," from libra "pair of scales, a balance" (see Libra). Meaning "characterized by slowness in decision, consciously unhurried" is attested by 1590s. Related: Deliberateness.
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.