late 14c., "having no deficiency, wanting no part or element; perfect in kind or quality; finished, ended, concluded," from Old French complet "full," or directly from Latin completus, past participle of complere "to fill up, complete the number of (a legion, etc.)," transferred to "fulfill, finish (a task)," from com-, here probably as an intensive prefix (see com-), + plere "to fill" (from PIE root *pele- (1) "to fill").
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.
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Definitions of completely
so as to be complete; with everything necessary;
the apartment was completely furnished
he had filled out the form completely
to a complete degree or to the full or entire extent (`whole' is often used informally for `wholly');