Generally used when someone backs out of something at the last minute. Sometimes used when complaining about previous arrangements made and then had plans changed.
To commit a "jambro" you must make some sort of complaint and then retract from participation.
Act of: leaving a man behind. To be in a "jam", "bro"
Related to: Janston and Sandy Vagina
"Hey, you are gonna play catcher now, I'm going to play second base."
"Hey man! NO WAY!!! I didn't spend $7 at the batting cages for nothing, I'm out of here man, play without me!"
"Oh man, don't pull a jambro".
2: Using Jambro:
"Come on man, we need one more player on our team! We are one short!"
"Naw man, I don't like to play football, I am going to go play WoW"
"Aw, don't pull a Jambro"
Also see Sandy Vagina