1) A song by All American Rejects
2) A song by Christian band Pillar
3) How you refer to a secret relationship, usually one in which some form of sexual contact has occurred.
1) "Dude, Dirty Little Secret is playin' on the radio!"
2) Dirty Little Secret is one of Pillar's best songs ever!
3) Jane: So, wanna come round later to continue our Dirty Little Secret?
Bob: Damn straight.