1530s, "a spark," Scottish, from Gaelic spong "tinder, pith, sponge," from Latin spongia (see sponge, n.). The sense of "courage, pluck, mettle" is first attested 1773. A similar sense evolution took place in cognate Irish sponnc "sponge, tinder, spark; courage, spunk." Vulgar slang sense of "seminal fluid" is recorded from c.1888.