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1.
One who tries to 1-up what you do.

A person who is trying to live YOUR life like it's theirs.

A Hater.
You: Hey, I'm going on tour with my band this summer on the West Coast!

Them: I'm going on a WORLD wide tour with my band.

You: *thinking to self* I swear he is such a life jacker.
by Karma Eve March 11, 2010
 
2.
Something or somone that prevents you from having a social life.
Guy 1: Man, ever since I started playing basketball for the high school team I've haven't done anything but eat sleep and play ball.

Guy 2: Yep sometimes sports can be life jackers.
by QMILLZ21 July 16, 2009