late 15c., "of the nature of an agreement," from Late Latin conventionalis "pertaining to convention or agreement," from Latin conventionem (nominative conventio) "a meeting, assembly; agreement," noun of action from past-participle stem of convenire "unite, be suitable, agree, assemble," from assimilated form of com "with, together" (see con-) + venire "to come" (from PIE root *gwa- "to go, come").
Meaning "of the nature of a formal meeting of delegates" is from 1812, now rare; that of "established by social convention, arising out of custom or usage" is from 1761; sense of "following tradition" is from 1831. Of weapons, "non-nuclear," from 1955. Related: Conventionally.
1896, reub, from shortened form of the men's proper name Reuben (q.v.), which is attested from 1804 as a conventional type of name for a country man.
late 15c., "a covenant or agreement," from French convenance "convention, agreement, convenience," from convenant, present participle of convenir "to come together; join, fit, suit" (see convene). Meaning "conventional propriety" is from 1847.