early 14c., "unmarried," from Old French sengle, sangle "alone, unaccompanied; simple, unadorned," from Latin singulus "one, one to each, individual, separate" (usually in plural singuli "one by one"), from PIE *semgolo‑, suffixed (diminutive?) form of root *sem- (1) "one; as one, together with." Meaning "consisting of one unit, individual, unaccompanied by others" is from late 14c. Meaning "undivided" is from 1580s. Single-parent (adj.) is attested from 1966.