"copulation, sexual intercourse," 1848, scientific use of Latin coitus "a meeting together; sexual union," past participle of coire "to come together, meet," from assimilated form of com "together" (see co-) + ire (past participle itus) "to come, to go," (from PIE root *ei- "to go").
In Middle English nativized as coite (early 15c.). Coitus was used in English in general senses of "meeting, uniting," and also in reference to magnetic force, planetary conjunction, etc., before the sexual sense came to predominate.
Coitus interruptus, "sexual intercourse in which the penis is voluntarily withdrawn from the vagina before ejaculation, for the purpose of avoiding conception," is from 1886 (from 1885 in German publications). Coitus reservatus in reference to prolonged copulation by deliberate control is from 1890 in English (1880 in German).