Sounds like the drain for the condensation that occurs on the evaporator core. Comes out of a nipple that has a rubber hose that runs down and out of a grommet on the floor (on the passenger side). This would be the culprit if you run the a/c compressor when you run the defroster.

This is a common problem as people's feet and vibration, in general, will work the hose off over time. -scott