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1.
The act of laughing at your own joke prior to delivering the punch line.
david letterman committed premature ejokulation last night.

dude thinks his jokes are so funny he prematurely ejokulates.
by spencedog April 11, 2009
 
2.
Attempting to make a joke before all the key parts that are necessary to make the joke funny are there; often with the hope that the missing parts will form by the time they are required.
Person 1: That was uncool.
Person 2: What's uncool is your... ... um... damn, premature ejokulation.
by dh718 March 26, 2010