Is it ensure, insure, or assure?
early 15c., "to join," from Latin copulatus, past participle of copulare "join together, couple, bind, link, unite," from copula "band, tie, link," from PIE *ko-ap-, from *ko(m)- "together" + *ap- "to take, reach." Sexual sense attested from 1630s. Related: Copulated; copulating.
copulate cop·u·late (kŏp'yə-lāt')
v. cop·u·lat·ed, cop·u·lat·ing, cop·u·lates
To engage in coitus or sexual intercourse.