[true_blue_texican]

I know, off is a preposition.

And while it is a preposition, prepositions also often serve as adverbs, and in this case "off" modifies HOW you are laughing. You're not just laughing, you're laughing your ass off! :-)

[Hilltop]

I think you dangled your participle, it is the object of what?

:)

[TSchmereL]

Never ending a sentence with a preposition is a rule up with which I will not put.