Jackoffs. Just like how the plural of bird is birds.
Actually, that looks correct. Here's an interesting twist though. What if the nound is not condensed? That is, what if "jack off" is used as two words but indicating a single entity? Which, then, of the following would be correct?
1. You jack off's are being so silly.
or
2. You jack offs are being so silly.
The original usage, which lexylit objected to, involved this sort of use of "jack off" as a noun. That is, it was used as two-word designation of a single entity, not a single-word designation ala jackoff.
Yes, arguing about grammar is silly. But if one is going to be clever and witty and use grammar naziism to come off as sophisticated then one ought to be prepared for a battle of nitpicking.