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1) To extract the fun out of something, to render something boring and or painful, to ruin something
2) A keyword in the LISP programming language
John: Let's play with the ball at the beach!
Mike: We can't, we have to study for the test tomorrow, and we have no hand-eye coordination, we're engeniering students remember?
John: You just defun my day. You defun everything.
by pr0gramm December 28, 2013
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to kill happiness slowly by making rules or changing subjects to current things people hate, or the process of being a killjoy.
When Bill started to take peoples feet off the couch and talk about Iraq, he started to defun the party.
by John Westbrook July 01, 2008
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