the action at which some one, who is nervous, clenches their butt cheeks in anticipation of a specific anxiety/nerve inducing event. -Walking is sometimes difficult, although it does make your ass look out of this world.
Bob:"Hey Todd, how's it going"
Todd: "Not so good, I have a huge presentation today and I have a serious case of TBS,I'm so nervous."
Bob: "Ya you are walking pretty funny right now, but your ass needs space boots, cause it's out of this world."
Bob: "Wait, what's TBS?"
Todd: TBS: Tight Bum Syndrome.
Bob: "Oh ya, that happens to me all the time, actually it's happening right now, give it a feel.