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1.
Someone that only thinks about themself. It's basically calling someone selfish, but more redundant.
Gary: Dude, can I have some cigarettes?
John: No! They're all mine! Fuck off!
Gary: Dude, fine. Y'don't need to be a selfish shellfish about it. Fuck.
by YapmelkXela October 14, 2009
 
2.
A shellfish that won't share a thing
Hey Larry I just ripped off that dudes claw.....Yeah give me some, nope. You asshole you are a selfish shellfish....

Mr. Crabs: I got me some more money.

Sponge Bob: My Crabs can I have a raise, Nope sorry Sponge Bob.

Plankton: If you can steal Eugene Crabs recipe. If you can get me that recipe Spongebob I will give you a dollar.

Sponge Bob: Oh sorry Plankton, I couldn't do that to Mr. Crabs, he is just a selfish shellfish.
by Looney Larry January 31, 2010