1) the result of wankin' it in the shower; the jizz runs down your hand, and gets stuck to your knuckles
2) a very unintelligent individual, or someone who happens to have said something stupid in the recent past
1) Goddammit! I hate when I come to class and still have cum knuckles... should've cleaned those bitches after jackin' it this morning
2) "I thought baseball was only seven innings." "Shut up, cum knuckles."