A phone call by a friend to another friend at a predetermined time. Mostly used in bad situations, first dates, or when someone goes somewhere they simply don't want to be.
Most times, a lifeline is used to show that one friend is in distress, when he's actually chillin at home with two beers, one in hand, and one waiting for the other friend. The distress call is what gives the friend his opporunity to leave, and also gives the impression to whoever is litening to the phone call that it's serious.
The lifeline remains the most efficient, yet most underused method of ditching bad dates. Sometimes hard to coordinate, lifelines prove their worth in effectiveness when your friend's girlfriend doesn't get pissed (and consequently, stop putting out) at your friend when you ditched her best friend on a date.
"Dude, Steve, I got a date with Jessica tonight at 7, give me a lifeline around 7:45, and then I'll meet you at your pad."
A person you can always depend on to help you in a really messed up situation where you need a quick excuse/escape.
This person will always have that six sense to lie in favor of you when asked about your current location (sometimes without you giving them any real hint about it).
This person also happens to be in the right place at the right time when you're in trouble.
"Dude, that blind date is horrific! I need to call my lifeline."
"Thanks, man! You're a lifeline!"
In situations where you're running late getting into work:
Boss: "Do you know if your name came into work today?"
Lifeline: "Uh... Yeah, he came in an hour before I went on lunch break. He's somewhere around here."