To fall asleep from exhaustion or from drinking too much; to faint
When we were in college, we used to drink until we passed out.
2) Tom was so tired when got home that he passed out on his bed with all of his clothes on.
'Pass' refers to
movement from one place to another, and 'out' refers to someplace other
than here. When you 'pass out' you move from normal consciousness to
unconsciousness, somewhere out there in the dark.