[fvirexi]

It's not based on stereotypes of Texas and Texans, but rather of the Wild West - as in the location of cowboy movies. There exists equivalent phrases refering to the Wild West directly, and at one point it seems "Texas" became (or perhaps already was) synonymous with "the wild west".

Speculation: in addition to cowbody movies, it probably has something to do with the norwegian book series Morgan Kane[1] from the 60s, as the phrase is mostly heard from people from around that time and before. [1]: https://en.wikipedia.org/wiki/Morgan_Kane