Acronym for "been there, done that, got the t-shirt."
Means you have already done the thing suggested, either to the point of mastery or disgust.
"Wow! That roller coaster looks pretty scary."
"bt, dt, gtts." /puffs out chest/
"Hey, you should totally go out with Anders. He's pretty hot."
"No way. bt, dt, gtts. He's an old fart."