When you try to jack off but you start to lose your boner because you can't think of anything or you lose your horniness.
John tried to jack off but he failed because he got boner-block.
Original definition circa 1987
When a really hot chick is standing next to you in a year book picture and you want that hard-on captured for all eternity for her to always remember you by, and the person in front of you with a full head of hair blocks the massive erection, boner block.
Another derivation: Anytime you are posing for picture with a really hot chick, and the massive erection is obscured from the final photo, but there is at least two dirty grins on their faces
Billy: Gee I hope this massive erection I have right now makes the school yearbook, and this girl's douche bag boyfriend's see where her hands is.
Tom:Dude, how could anyone possible miss it?
Two months later.
Tom: Billy you totally gotboner-blocked, Lisa's big head of hair totally blocked the shot, but not everybody's grins.