Rhyming expression for use when you really didn't know something, and were maybe even the last person on earth to find out. Goes to show you really do learn something new every day.
'You mean you can't get pregnant by sharing a facecloth? Fuck my hat, I didn't know that!'
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi