Someone who does not leak information.
"It's OK, you can tell me, I'm watertight."
by Penza March 02, 2004
Expession used to describe a female involved in 4-way sex with three men. One man orally, one man anally, one man vaginally. This would fill all of the availible popular hole, thus making her "water tight".
Last night we had that bitch water tight!
by Lars from Mars September 05, 2004