The combination of "cum" and "crumb". Usually created in the act of wiping the cum off from your manhood with a Kleenex, then immediately, or soon after, wiping off some crumbs (probably Nabisco brand Chocolate Chip Cookies with 25% Reduced Fat) off of a surface with the said Kleenex; creating Cumb. Not recommened for consumption, unless you're a chick. That is totally fine.
Dude, is that Cumb in your wastebasket?
Uh, you mean cum? Probably.
Nah, I think I see some cookie crumbs in it, too...