To leave, exit, opt out of or abandon an event, group or activityin a way that disappoints the other members.
#1
At a party:
Jimbo: It's getting late. I gotta go home. You're on your own.
Heinz: You're laming out on me again?
Jimbo: I'm tired.
Heinz: Okay.. but next time we go to a party, don't lame out on me.
#2
In a car:
Jimbo: Where's Heinz? I thought we were all going to Vegas tonight.
Steve: He lamed out.