Foss: To screw up royally.
In a Vampire game I ran, one of my players had country-bumpkin Gangrel named Grant Foss. Foss managed to screw up everything he tried in the first six sessions we had, and even once he got better, he became the joking scapegoat of everything that happened afterwards ("Foss, what did you do to the Temple of Set?" "Foss, what did you do to the sun?" "Foss, what did you do to the Hunter?"). Since then, Foss has become a verb in our group. When a long, drawn-out plan crumbles because the catalyst rolls a 1 on his Jump check and falls off the ship, he has really fossed things up. I've even gotten this to catch on at my office, though I haven't said what it means (they assume it's a way to avoid dropping f-bombs). Important Note: The player finds this term awesome, and encourages its use.
Dice Grenade: To use dice as a splash weapon.
My girlfriend is new to gaming, and plays a spellcaster/summoner in my Final Fantasy game. Being 14th level, she's casting 14d8 Fira/Blizzara spells. Most people would just roll the dice, but not her. She gathers up fourteen d8s and HURLS them into a box, creating a ridiculous noise that tends to echo. We used to not even have the box, back when she rolled d4s for Fire spells. However, soon she was rolling enough dice to send the d4s sailing, and we implemented the box (which doubles as a blast shield for those of us at the table).