The word "Mingebag" comes from the Half-Life 2 modification, "Garry's Mod" by Garry Newman. This word basically means a person that is new to the game and does not know how to play. Often, mingebags ask questions that are very simple to answer.
Why was this word created? Back in Garry's Mod 9, the name you would have set to your player by default would be "mingebag", until you changed it in the options, so everyone knew what to expect. Now, in Garry's Mod 10 that has changed. The players now decide who is a mingebag. Usually it's a little kid, someone who spams, or just one who doesn't know how to play the game.
Nobody really knows why Garry decided to use this random word.
Mingebag: "Hey guys! How are you floating in the air like that?"
Mingebag has been kicked by console. "Mingebag".