early 15c., "united or sharing" (in some activity), from Old French jointiz (adj.) "joined together, close together" and Old French joint (14c.), past-participle adjective from joindre "to join, connect, unite," from Latin iungere "to join together," from nasalized form of PIE root *yeug- "to join."
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 jointly from WordNet
in collaboration or cooperation;
this paper was written jointly
in conjunction with; combined;
Synonyms: collectively / conjointly / together with