Top Definition
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
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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