SPARk+SPARK=SPARK

The word spunky (first attested in 1786) means "having the quality of spunk (first attested in the early sixteenth century, and, nope, no connection to punk)". Spunk, meaning something like "spirit" today, took a metaphorical shift from its earlier meaning of "spark". This is a combination of two other words: spark, meaning "spark", and funk, an obsolete word meaning "spark". That may sound a little confusing, so let me clarify: SOMEBODY COMBINED TWO WORDS FOR SPARK TO MAKE A THIRD WORD FOR SPARK. Obviously the definition has evolved from then, but that's literally what happened when the spunk first came about. Funk, a pretty basic word, went through alterations all the way back to Proto-Germanic, as funke, fonke, funca, fanca, funko, and fanko, continuously retaining its definition. That is, until we trace it to Proto-Indo-European peng, "to shine". I'll save the word spark for another occasion (the winning definition survives). Just take in this crazy portmanteau.