An aciditty is a song written under the influence of acid. Or LSD. Although it's pretty much self-explanatory, for duds who haven't heard one, remember Lucy In the Sky?
Dude 1: Dude, have you heard this song, In the Garden of Eden?
Dude 2: Huh? Dude, what have you been sniffing? It's Inna-Gada-Da-Vida, the famous aciditty composed by Iron Butterfly
Dude1: Aciditty? Now what's that?
Dude2: (sighs) Try some acid and say In the Garden of Eden