[nascarnation]

https://www.roadiemusic.com/blog/a-guide-to-guitarist-slang-terms-you-need-to-know/

[gitmo]

Thanks.

[real saxophonist]

The link only partly explains it. It come from the Woody Herman big band, which at times was called Woody and the Woodchoppers.

So the guys in the band started calling their instruments 'axes'.