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
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.