late 13c., "two of the same kind or class connected or considered together," especially "a man and a woman associated together by marriage or love," 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."
From mid-14c. as "that which unites two." In electricity, "pair of connected plates of different metals used for creating a current," from 1863.
c. 1200, "to link or connect, as one thing with another," from Old French copler "to couple, join together," from cople (see couple (n.)). Meaning "unite in marriage" is from mid-14c.; that of "embrace sexually, copulate" is from c. 1400. Related: Coupled; coupling.