Stealthsturbation is masturbating in a scenario that would require stealth in which you must to extremely aware as well as silent. Note that all parts of the masturbating must be stealthy (gathering cleanup materials, the initial act, and cleanup).
Situations of stealthsturbation include when your parents are next door, when your parents or significant other are going to be home soon, or when your sibling is just a room away.
To be a "tiger underwater" means to be really, really angry (or "super-pissed"). Taken from the actual event of a tiger being underwater. Upon searching "tigers underwater" on Google images, you will find that every single tiger underwater is abnormally upset with everything around it.
Ways to use it include:
"Dude I'm a tiger underwater right now"
"Yeah, he's being a tiger underwater"
"Dude, quit being a tiger underwater".
"You know, If I was just playing GTA4 because I was bored and I got the Red Ring of Death, I would have been pissed. However, I got the Red Ring when I was about to play a game I've been waiting months to play, so you could say I'm a tiger underwater."