Badgie can really mean just about anything. However the word is usually applied as an acknowledgment to what someone has previously said, often is followed by a question mark. It can also be used when something happens or is said that is meant to be taken seriously in which case you don't take it seriously you simply reply "Badgie" or "badgie?"
If used after a statement and the person it was directed to replies "badgie" or "badgie?" this shows that the person it was directed to agrees with/understands what was stated. In using it this way literally anything could be implied.

A usually insulting word that can be applied to literally any situation.

This word is not only limited to having an insulting meaning, it can mean any one of an array of words.

Friends can use it to refer each other, then turn around and say it to someone they absolutely hate.

'Badgie' is commonly mistaken for 'Budgie', but getting these words mixed up and using them in the wrong context may result in severe injury, even death. It is highly advised that you DO NOT use this word unless you truly understand the power and gravity that it holds. Once unleashed, it cannot be undone.

It is the most versatile word in the English language, 'Badgie' can mean anything from wanker, douchebag, DLCS and fuckwit to mate, friend, gun-slinging hoe-bag, reece, dave, etc.