Making a "queef" style noise with your penis. Involves getting the air in your urinary tract to form a vacuum - letting the air out with a PRRRRRUITT-sound.
A deef can mean a few things. But here's how cool people use it...
1. The act of ruining a fun situation or to increase the amount of faggotry within a given area. Like saying "man you deefin' up the place" or "man, jefferey be str8 deefin'"