"copulation," 1713, scientific L., from coition (1610s) "sexual intercourse" (also in coitu), originally "coming together" (1540s), from L.L. coitionem (nom. coitio), from pp. stem of L. coire, from co- "together" + ire "come, go," from PIE base *ei- "to go." Used in Eng. in general senses of "meeting, uniting," and also of magnetic force, planetary conjunction, etc., before sexual sense came to predominate.