According to Daniel Cassidy’s ridiculous book, How The Irish Invented Slang, spunk comes from the Gaelic or Irish word sponnc. Cassidy is completely right about this. It does come from Gaelic or Irish. Sponnc (or sponc in modern spelling) does mean tinder or material to light a fire and the metaphorical use of this to mean fiery personality, courage or semen is quite clear.
So, does this mean that I have changed my mind about Daniel Cassidy and that I have decided to support him? Not a chance!
You see, Cassidy says that the OED claims that spunk is of uncertain origin and possibly related to punk and funk. So a naïve reader who believes in Cassidy’s integrity would accept that this is the view of all the English dictionaries. How stupid those dictionary people in their ivory towers are! There’s a word in Irish with the same meaning and these people refuse to believe that there’s a connection! Hooray for Daniel Cassidy, the man who gave Irish its rightful place in the dictionaries! The only problem with this is that most of the other English dictionaries already give the Gaelic or Irish derivation of spunk. They did so a long time before Cassidy started on his crazy project (and I imagine that the compilers of the OED also knew about this theory but decided that there was insufficient evidence that the word didn’t derive from the Latin spongia through some other route.)
Don’t believe me? Here’s Merriam-Webster:
“Scottish Gaelic spong sponge, from Middle Irish spongc, from Latin spongia sponge.”
And Collins’ Dictionary also derives spunk from Latin spongia through Scottish Gaelic spong. It’s available online!
So, how did Daniel Cassidy miss these Gaelic origins from the ‘Anglophile dictionary dudes’? Well, I think it’s a pretty fair bet that he didn’t miss them. Cassidy deliberately decided not to put them in because they spoil the nice little fantasy that he was single-handedly fighting the fight for Irish against a coterie of Anglophile dictionary compilers.