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1.
The comparative term to the male "cock block." It means to effectively stop a female companion from getting laid.
I was gonna bring this hot guy back to my apartment, but my bitch roommate decided to pull a clam jack.
by Lon October 11, 2005
 
2.
When a woman interferes with one's attempt to hook up with someone. Similar to "cockblock".
"He was totally into me, but then that bitch showed up and clamjacked me."
by Whereswill January 23, 2007