noun; a person with whom you share the common connection of having both slept with the same person, creating an awkward, familial-like connection as a result of you having shared the same sexual partner, albeit at different points in time. It is typically more preferable to be "older" of the fuck cousins, as you engaged in the sexual activity with the intermediary partner prior to the "younger" cousin. Moreover, the newly-discovered fuck cousin-connection transcends and supercedes all other connections you may have previously had with the fuck cousin, whether it be family, friend, or complete stranger.
1) Dude, you know how you fucked Jackie a couple of years ago? Well, Andy just told me he fucked her last week! Looks like you've got yourself a fuck cousin!
2) My creepy Uncle Eddie boned the same chick as me?!? I guess he's no longer my uncle, now he's my creepy Fuck Cousin Eddie.