A small, usually unintentional shower of spittle that jumps out of one's mouth directly from the salival ducts beneath the tongue.
The ability to guppy on command can be achieved through practice, by rolling the tip of one's tongue toward the back of one's mouth. In popular usage during the nineties, especially among teenage boys.
My teacher Ms Hoover starting yelling at me not to chew gum in class, and when I went to take it out of my mouth I accidently guppied in her eye.
The sexual act of bringing your manhood close to a girls face so she tries to mouth it, but you pull away at the last second leaving her with a guppy mouth. Repeat as necessary.
I made that chick a guppy last night, she got pissed and left but the look on her face was priceless.