Stands for 'Lights Out, Missionary Position'. Used to describe a person with unimpressive sexual habits. Most likely, this is an attractive girl who doesn't need to perform to satisfy men. Can also be used to describe how sexual desire wanes in a long relationship.
"Hey Dave, how's your L.O.M.P.? When's the last time she gave you head?"
"I need to head down to the sex toy store because we've been getting kind of LOMPy."