An acoustic guitar. "Git" is short for guitar, while the "box" refers to its hollow body.

Usage of the phrase can be found in an episode of "The Twilight Zone" called "The Passersby" from 1961. In the episode, a Civil War veteran refers to his git box, suggesting that this phrase dates back to at least the mid 1800s.
Just sit back and relax while a strum a few on my git box.
by Brewchief November 14, 2010
A guitar but when owned by someone who thinks they are cool. From sk8r punk, Blink 182 ect.
Hey Billie, what up with the new strings on ya git box!!!!!!1111!!!!!
by Andrew Shaw October 26, 2004
the slang term for a "vagina" signifying the utter stupidity of the word, but it also has another meaning. The "Git" part means that you want to "get into", while the box simply signifies the woman's pussy.
Holy Shit! That Bitch has got one dandy gitbox! I'm 'a take a look at it!
by jeff March 2, 2005