1. A completely ignorant, childish person with no manners.
2. A person who feels justified in their callow behaviour.
3. A pubescent kid who thinks it's totally cool to act like a moron on the internet, only because no one can actually reach through the screen and punch their lights out.

A stupid person; it refers to the lack of surface area on an individual's brain. The general thought is that the more surface area (wrinkles, creases, etc.) a brain has, the smarter the person is. Conversely, a person with a smooth brain (no wrinkles) has less surface area and would therefore be stupid.

Total and utter tosser who is incapable of doing anything other than annoying people, and not in a way that is funny to others. Best used idly. e.g. "Git."

By definition it's a "rotter" or a despicable person. In reality in English it's a colloquialism that affords itself to a more a luckier fellow. Much like a 'bugger' whilst the roots of the word are offensive, the actual use is endearing. A lucky 'git', a wealthy 'git' and a sodding 'git'... when used right is still a mate. A friend.

It's more positive these days.

The origins are from the word 'geat' which were a Scandinavian people living in Götaland, land of the Geats, currently within the borders of modern Sweden. The name of the Geats lives on in the Swedish counties of Västergötland and Östergötland, the Western and Eastern lands of the Geats, as well as in many toponyms.

Git is a free & open source, distributed version control system.

a person that is useless, troublesome or annoying, a fool.

From the english dialect "Git" or "Get" meaning "Gate". When using foundry sand to cast iron objects the access point for the metal, that is the funnel shape that lets the molten iron into the sand-mould was called the gate. This gate was of no use and would have to be removed from the final item and the item carefully finished to remove any blemishes caused by the gate. Thus the term "Useless gate (git)" was coined.