Generally, slap means “to strike sharply with an open hand” or refers to such a hard blow so delivered. But what could this action have to do with groovy tunes?
Well, consider this: we call an extremely successful song a smash hit. And a song that we personally love? It’s our jam. Smash hit, jam, slaps—these terms, and many more, use direct, smack-dab force, contact, and motion as metaphors for excellence.
As for slap itself, the association of the word slap with great qualities isn’t brand spanking new, exactly. It was an adjective for “first-rate” in the mid-1800s and an adverb, meaning “excellently,” even earlier, in the mid-1700s.