Is it farther or further?
late 13c., from Old French cople "married couple, lovers" (12c., Modern French couple), from Latin copula "tie, connection," from PIE *ko-ap-, from *ko(m)- "together" + *ap- "to take, reach." Meaning broadened mid-14c. to "any two things."
c.1200, from Old French copler, from cople (see couple (n.)). Related: Coupled; coupling.