early 15c., from Anglo-French espaundre, Old French espandre "to spread out, pour out," from Latin expandere (see expand). The notion is of a "spreading out" of fish eggs released in water. The meaning "to engender, give rise to" is attested from 1590s. Related: Spawned; spawning.
early 15c., from spawn (v.); figurative sense of "brood, offspring" is from 1580s.