It happens from experiencing jonas brothers concerts, seeing nick jonas, kevin jonas or joe jonas shirtless, sweaty, or wearing muscle shirts. May also occur when you stare at the jonas crotches for too long (especially a picture of the joner.)
Having a jonasgasm makes you want to throw a jonas up against a wall and rip off their purity rings.
jonasgasm: jonas orgasm.
Girl 2: OMJ! look at his muscles!!!!
Girl 1: OMJ IM HAVING A JONASGASM!