The JP-tron: another name for confusion or anything else that makes character behave randomly.
Origin: JP, one of our players, tend to have his character behaves strangely and incoherently. Like when his vampire characters in a VtM game locked his house's cave's door, threw the key in the sewers, and then started to break the wall beside the door to pass through...
Intellectuel souffreteux ("sick and frail intellectual"): a kind of character concept, with high mental scores and low physical scores.
Origin: A V:tDA game was started, and the GM said we would begin as (still-living) mercenaries during a conflict. One of the players created a character that was an asthmatic young girl disguised as a boy to escape unwanted marriage, and with enough engineering knowledge to craft and use her own crossbows. I said oh, encore un intellectuel souffreteux dans une campagne pour bourrins ("gee, another sick & frail intellectual in a bully campaign"), and it stuck.
"Cocorico!" ("Cockadoodledoo!"): What clerics of Lathander shout during their daily prayers.
Origin: In character mocking of a CoL PC.
"His name starts with an A": He's a villain.
Origin: In our first D&D campaign, all the villains had a name starting by A.
"Pious knight" (subtype of the sick & frail intellectual): a young and delicate sorceress without any knowledge of battle.
Origin: During an Ars Magica campaign, one of our players (the aforementioned JP) had his maga enters a labyrinth where magic was impossible and in which, it was said, awaited trials that could only be passed by a pious knight. The maga walked bravely in, turned around the first corner, and came back 10 seconds later, crawling on the ground, a gaping wound in her flank, a lazy skeleton following her ssslllooooowwllly with a big scimitar.
"To be unhappy": To be hit badly.
Origin: One of the GM in our group had the habit of commenting on successful attack/damage rolls by saying the enemy "is unhappy". A slightly damaging blow: "he's kinda unhappy." A grievous blow: "he's starting to be really unhappy, now."
It was so silly it stuck.