Almost everyone coins or uses expressions that make sense to only a few people. What word or phrase do you use most often that you have to explain the meaning of to others?
I definitely say "IRL" and people don't get it. It means "in real life," as opposed to on the intarwebs. I also find myself saying "WTB [x]" which comes from World of Warcraft. The acronym stands for "want to buy," and people say it in trade channels when they're looking for an item, but people also use it and the opposite, WTS (want to sell) sarcastically - if someone talks in a general chat and spells horribly, someone else might snarkily comment "WTS english skill PST [please send tell]". So when I want something, I occasionally say "WTB sleep" or something of that nature.