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1.
When two women have had sex with the same man/men, they become clam cousins. This is a bond that can never be broken...unless they get jealous.

Variation: instant clam cousins - the resulting relationship due to a menage a trois.
"You see those two chicks over there? They're instant clam cousins. You know what that means? Oh yeah dog! Threeway!"

"My best friend Jullie and I just found out we're clam cousins!"
by monkeyboy72 November 14, 2009
 
2.
When two women have had sex with the same man/men, they are Clam Cousins. It is the same concept as two men becoming Weiner Cousins or Eskimo Brothers after sleeping with the same woman/women. Except, unlike men, who become closer than brothers after discovering their bond, women who are Clam Cousins must become sworn enemies.
"Did you hear that Angelina found out that her boyfriend used to sleep with her friend Jennifer back in the day?"

"Uh-oh. Now they can't be friends anymore. They are Clam Cousins!"
by realschmiel82 November 18, 2009