This phrase basically means to take something to the next level in a ridiculous way. It was first seen years and years ago on MTV during a stupid show where people on the street were asked if they would drink a beverage out of their shoes. the point was to try to get them to do it for the least amount of money. During one such segment, two dudes in the background were watching as others yelled out low numbers. They had the genius idea to take it to the next level and chant "Do it for free!" over and over. People knew it was stupid, and therefore ignored them.
Host-"If I give you 50 bucks, will you drink this out of your own shoe?"
Contestant-"I dont know man, maybe for 100 bucks"
Dudes-"DO IT FOR FREE, DO IT FOR FREE, DO IT FOR FREE!"
Host-"heres a hundred bucks"
-embarassed dudes laugh amongst themselves after being totally ignored-