cheeky nut
The act of quickly pulling one or both of your testicles out to surprise a mate. Leaving it out for only a second or two, it’s a quick flash, and that’s what makes it cheeky.
Matt: turn around
Me: *turns around*
Matt (with one testicle hanging out of his fly): cheeky nut
Me: *turns around*
Matt (with one testicle hanging out of his fly): cheeky nut