2. In colloquial conversation, an event which is surprising or which seems unlikely but is (supposedly) true.
a man with father, grandfather, and great-grandfather all with the same birthday, or
Hindu priest who can drive a six inch metal spike into his nose all the way into his brain.
Def 2 examples:
Believe it or not, we are having twins! AGAIN!!
Believe it or not, she was choking, I was just performing the Heimlich maneuver!!
Believe it or not, I got the highest mark in the class!