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"