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1530s, "a spark," Scottish, from Gaelic spong "tinder, pith, sponge," from Latin spongia (see sponge). 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.
To ejaculate semen; come: the filthy pigs spunking into women (1970s+)
[apparently fr Celtic spong, ''tinder, touchwood, punk,'' fr Latin spongia, ''sponge''; apparently semantically fr a resemblance between semen and a spongy excrescence found on trees, in which sense the word is found in British dialect]