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The act of attending a university lecture for a class you do not take.
Derivative of gate crash

Reasons behind this behaviour vary... One could lecture crash to keep a friend company, to hide the fact that they have no one to hang out with during a break, to try before you buy, or maybe just because they find the subject interesting.
"Hey Clementine! What are you doing here?! I didn't know you took psych 101!"

"Hey Fatima, nahh I don't, I'm just lecture crashing"
by RoboBear September 20, 2011