An erection that occurs every time you are exposed to a certain, seemingly unrelated stimulus.
For instance, getting an erection every time you see a hot chick is not a pavlov's dong, but getting an erection every time you see a trumpet would be.
Pavlov's dong is usually caused by association. If you had this really hot girlfriend who loved donuts and she broke up with you, the association of your old girlfriend is what would cause a pavlov's dong with donuts.
"Dude, you're in a room full of guys and you just got a boner."
"No homo man, it's just a pavlov's dong."