Term: Eating the Clue
Usage: 'Wow, they really ate the clue on that one.'
Meaning: To destroy the clue or MacGuffin needed for an adventure.
Source: A vampire game where they needed to talk to the victim off a magical attack, and instead of interrogating the wounded NPC... ate him. I ran across the term being used online once - attributed to 'some guy in Maine'. I was so proud.
Term: Blow Mighty Dodge
Usage: 'And now to use my Blow Mighty Dodge skill!'
Meaning: To be hit against all the odds of probability, or to miss, again against all the odds of probability.
Source: A Warhammer Fantasy Roleplay game, where one player commented that his Dodge Blow and Mighty Blow skill had combined to form Blow Mighty Dodge...
Term: Spot Obvious
Usage: 'I blew my spot obvious roll!'
Meaning: A player (not a character) has failed to notice something out in the open. Best example - player lifting his Player's Handbook to look under it, while asking 'has anybody seen my Player's Handbook?'
Source: Pulling the book from his hands and giving it back saying 'I make my Spot Obvious roll!' Spot Hidden was a skill in just about every game by Chaosium, Spot Obvious is be extension its evil twin.
Term: The Mighty Forces of Choas!
Usage: 'You must face The Mighty Forces of Choas!'
Meaning: To mispell or mispronounce something in a dramatic fashion. Alternately to dramatically introduce something that does not actually work well in play.
Source: A Warhammer 40,000, where one of the armies was The Mighty Forces of Choas! Complete with exclamation point. The player then managed to lose every single battle he was in... After the spelling error was pointed out to him he pronounced it the way he had spelled it, and was a very good sport about his losses. He managed to lose against an Imperial Guard player who's average roll was 2 (when rolling a D6).
Term: He's doing it again!
Usage: 'He's doing it again!
Meaning: To do something really stupid, repeatedly.
Source: A call of Cthulhu game set in the 1600s, where the player who ran the captain of the ship would always close to hailing range, no matter what the mission at hand was. The result was three ships in a row being sunk out from under the party... He really hated the term, and eventually picked up on the fact that he was doing something dumb.
Term: I Bring the Metal Detector
Usage: 'You guys get ready for combat, I'll be along later with the metal detector.
Meaning: Can't you guys do something without screwing up at least once?
Source: A Modern Call of Cthulhu game, where I followed up after several confrontations with the cultists with a metal detector, picking up spent brass and getting rid of the bodies that the rest of the party had 'hidden' in some stupid fashion (like up in the branches of a tree.) It caused an encounter with a policeman in real life once, as I harrangued one of the other players with 'I can't believe you guys didn't wear rubber gloves while loading your guns!' while sitting at a table in a fast food restaurant, about 10 feet away from the policeman in question... My character had an amazing ability to be nowhere near where the combat was taking place, because I was always careful to not let combat happen if I could help it, and the other characters would get into gunfights when I wasn't around. (Healing in CoC takes a good long time, and it is easy to get dead.)
The Auld Grump