mid-14c., "moderate in desires or actions, temperate, restrained," especially "abstaining from strong drink," also "calm, quiet, not overcome by emotion," from Old French sobre "decent; sober" (12c.), from Latin sobrius "not drunk, temperate, moderate, sensible," from a variant of se- "without" (see se-) + ebrius "drunk," of unknown origin. Meaning "not drunk at the moment" is from late 14c.; also "appropriately solemn, serious, not giddy." Related: Soberly; soberness. Sobersides "sedate, serious-minded person" is recorded from 1705.
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.