Also spelled as zerbert, it's a term popularized by Bill Cosby back in the 80s. You know how you can make fart noises by pressing the palms of your hands to your mouth and then blowing hard, causing nice flatulent sounds to come through the space and vibrating the skin of your mouth? A zerbert is that, only applied to someone else, usually on the tummy, and preferrably someone you have a physical relationship with (a stranger or casual friend probably wouldn't appreciate it too much).