Throwing a monkey is a sacrifice you are making for your friend and can be accomplished in many ways. Throwing a monkey for someone is a highly respectable act and lets people know that you are unselfish, reliable, and when it comes crunch time you can come through in the clutch. You will likely get at least one favor in return for successfully throwing a monkey.
This term is in relation to throwing a monkey in Call of Duty Black Ops Zombie mode, which in dire situations you throw to force the ugly zombies that you know are about to be a potential problem or that are annoying you to remove themselves from your immediate location and follow the monkey so you can do something satisfying like hit the mystery box or upgrade your ray gun. Monkeys are often thrown when you need to revive a friend to help him get back into action.
Garrett: "Yo dude I think I might have that hot ass girl in the bag I've been talking to tonight, but her ugly ass friend won't leave her side so I can really start spitting my A game and seal the deal, can you throw a monkey for me?"
Foster: "Yeah dude, I'll go talk to her and get her to follow me to the other bar upstairs and buy her some drinks."
Garrett: "Thanks man I owe you one"
Foster: "Yo dude I need you to throw a monkey for me man asap"
Garrett: "I got you dawg, you helped me out last time. I'm gonna slip my wallet in her coat pocket secretly and tell the bouncer i saw her steal it off of the counter and make sure she gets kicked out, no worries man I got your back"