A person, usually a woman, who avoids risque sexual activities, especially those involving a "backside" nature. The phrase derives from the tendency of functional groups attached to t-butyl to avoid participating in chemical reactions due to steric hindrance. Often affectionately abbreviated as t-but (pronounced t-buit).
Guy: Man that nun is hot!
Other Guy: Yea but I'm sure that she's a t-butyl!
A woman on whom backside attack is impossible due to bulkiness. The hindrance resulting is a case of too much cushin for the pushin. So named for inability of a tertiary alkyl halide to undergo the Sn2 reaction.
Guy: Oh man, Oprah is so fine. I'd doggie on that all day.
OtherGuy: Dude, you never could, she's a total t-butyl.