The act of unconsciously typing nonsense while half asleep in a Messenger conversation. The nonsense is usually ridiculously misspelled words, or just complete gibberish.
Jenna: OMG Fred, what are you doing up so late?
Fred: PHusicdp aper die tmrowqqwr
Jenna: Are you sleep typing?
Jenna: Last week, I was talking dirty to Fred over messenger and he FELL ASLEEP.