The word 'spunk' entered English in the 16th century from Scottish and Irish English, where it originally meant 'a spark' or 'touchwood' (a fungus used as tinder). This sense derives from the Latin 'spongia', meaning 'sponge', via a hypothesized Vulgar Latin form, reflecting the porous, flammable nature of tinder. The metaphorical leap from a 'spark' to 'fiery spirit or mettle' is a classic example of semantic shift, where a physical attribute (the quick ignition of a spark) comes to represent a human quality (quickness of spirit, courage). The vulgar anatomical meaning developed later in the 19th century, representing a further, distinct metaphorical extension.

This word is a simplex form. It is a loanword and later semantic development from Scottish and Irish English, with its core elements not productively separable in modern English.