When you go to the bathroom during an instant messaging conversation and return without having to explain a delay in replying. Only applicable if the time you were in the bathroom is unnoticeable and you don't have to lie about your disappearance when you return.
Some people are easier than others to do this with; i.e. if a person is capable of talking for half an hour wanting a reply to make sure you didn't go on holiday, they are often easy targets for a stealth piss.
Person 1: "So how are you?"
*** person 1 walks off to go to the toilet without informing person 2 ***
Person 2: "I'm fantastic!"
Person 2: "I got paid earlier so I bought some really awesome shoes in town"
Person 2: "And then I bought like 5 new CDs"
Person 2: "Which I'm listening to at the moment"
Person 2: "Oh and I saw your friend from school"
*** person 1 returns from urinating ***
Person 2: "He said you should come out sometime"
Person 1: "Yeah I think I should sometime!"
(person 1 has performed a stealth piss, unbeknownst to person 2)
A method of urination (males only) in which an individual pisses around on the toilet bowl but NOT the water so as to avoid making any kind of audible sound
Ah shit, there's somebody in the next cubicle, better stealth piss