An organized date usually arranged by friends where the sole purpose is sex and/or getting over a failed/bad relationship. Should not be confused with rebound. A lay up may or may not culminate into a meaningful relationship.
Matt felt bad for James so he called Alice and had her arrange a lay up with one of her friends.
A coded word that brings attention to the fact that someone in the vicinity is making a move on the opposite sex (without being obvious). The move being made may be overt, subtle, physical, flirting etc. Checks are usually done to embarrass the parties involved.
Advanced users may make a more complex coding system such as check-plus, check-minus, j-check, s-check etc which may be user defined.
When Jay saw Frank talking up the hot new professor, he shouted, "Check!"