'opposite-gender' originates as a modern English compound formed from the adjective 'opposite' and the noun 'gender'. 'opposite' ultimately comes from Latin 'oppositus' (from 'opponere'), where the prefix 'ob-' (or 'op-') meant 'against' and 'ponere' meant 'to place'. 'gender' comes from Old French 'gendre' and Latin 'genus', where 'genus' meant 'kind' or 'type'.

'opposite' passed from Latin 'oppositus' through Old French and Middle English into the modern English adjective 'opposite'. 'gender' developed from Latin 'genus' to Old French 'gendre' and Middle English 'gender', shifting in sense from 'kind/type' to 'sex'. The compound 'opposite-gender' is a recent English formation used to denote the gender opposite to one's own.

Initially, the roots meant 'placed against' (opposite) and 'kind/type' (gender). Over time, 'gender' narrowed toward meaning 'sex' and 'opposite' retained the sense 'other' or 'facing', so the compound came to mean 'the other sex/other gender'.