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An abbreviation which stands for "Send Your Buddy Some Titties Day", which happens to be every Monday. This most glorious of days is celebrated by, quite literally, sending your buddy some titties whether it be by picture message, email, Facebook, etc..
Bro A- "Dude, why'd you post a bunch of pictures of tits on my Facebook?"
Bro B- "Its SYBSTD man.."
Bro A- "Oh... thanks buddy!"
by C-Los Fo' Shizzle February 18, 2011