When you awake from sleeping with a cold to find that you have drooled so profusely out your mouth during the night that your spit has turned to a white crust around your mouth with bits of sticky, white, congealed spit hanging off your lips. It also describes the terrible, terrible, taste that you experience upon waking up. Deep throat bonus if you wake up with a sore throat too.
"Shit, did somebody rape me in my sleep? Wait... it's just semen mouth."