A fweep is when air comes out of a penis.
A woman queefs, and a man fweeps.
For a fweep to happen, air must be pushed down into the shaft of the penis via the urethra. This air is condensed in the shaft of the penis near the balls, which is then forcibly pushed up, creating a "fweep."
"I was ready to go down on him, and then he fweeped right on my face!"
"Man, I was hanging with Marty the other day, and I heard something strange from his pants.. I think he fweeped!"