British Slang

Just to add my two penn'orth to this, Merriam-Webster gives the definition as "git n [var. of get, term of abuse, fr. get] (1943) Brit: a foolish or worthless person"

The probable use of the word as an insult likely originated with the use of "get" in the sense of offspring, and by the 16th Century, was being used in Scotland and the North of England to describe someone who was illegitimate, a bastard in the orginal and strictest sense of the word.

James Joyce uses the older spelling (and meaning) in Ulysses in 1922: "The bloody thicklugged sons of whores' gets!" ¹

Modern use is more general - it is a mild insult, often used in a jocular or even somewhat affectionate manner, as in "you daft git", but is often used to describe older people - Harry Enfield had two characters known as 'The Old Gits'. This contrasts with 'twerp', which is more associated with young people.

Interestingly, "get" seems to be used with increasing frequency as you move north through England and into Scotland, which to my mind, confirms this origin and usage.

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Albert Herring says - also note the Beatles' (or Lennon's specifically) "...and curse Sir Walter Raleigh, he was such a stupid get." (I'm so tired, white album).
Gorgonzola says re Git: There are several infamous lines from Monty Python, which I will not bore you by quoting.

¹ http://www.worldwidewords.org