Pronouced: Jay-pin - Short for Just Popped In (to my head)
This acronym is used after something is said that really came right out of left field (i.e. a random, often weird thought), bearing little or no relevance to the topic that may or may not have been at hand. Saying jpin acquits the person who said the randomness of their insanity for that instance, showing it to be completely unserious and/or comical in nature. Normally only applies to short dialogue.
A: What should we do for dinner tonight?
B: I think we can either do stirfry or mac and cheese.
A: Ooo, both sound awesome.
A: STIR-FRIED MAC AND CHEEEEEEEEESE!
B: You're a spaz.