Literally, What The Fuck Boner, a WTFB is an erection which develops at a seemingly inexplicable moment. It is characterised by a swelling of the penis, as well as a distinctive absence of arousing stimuli.
When a male gets a WTFB around a group solely consisting of other males, it is appropriate to announce your WTFB publicly, so as to avoid being labelled a fag.
"Dude, I was doing my maths homework when suddenly I got a WTFB. Wtf?"
"Woah, WTFB. Don't know why that happened around this group of athletic and sculpted men."