'opposite-sex' is a compound formed from 'opposite' + 'sex'. 'opposite' originates from Latin, specifically the word 'oppositus', where the prefix 'ob-' meant 'against' and the root 'ponere' (via past participle forms) meant 'to place'. 'sex' originates from Latin 'sexus', meaning 'state of being male or female'.

'opposite' came into English via Old French and Middle English (from Latin 'oppositus') and developed into the modern English word 'opposite'. 'sex' entered English from Latin 'sexus' through Old French 'sexe' and Middle English to become 'sex'. The compound 'opposite-sex' arose in modern English usage by combining these two established words.

Initially, 'opposite' had a literal sense of 'placed against' or 'facing', and 'sexus' referred to classification by biological sex; over time the compound came to mean 'the sex that is opposite to one's own' or 'involving members of different sexes'.