Variant of buey. Compare güeno, agüelo, güelvo, where /bwe/ becomes /gwe/. This mutation is considered substandard for Spanish. Interestingly, though, buey is derived from Vulgar Latin *boem, which derives from Latin bovem, which is a form of bōs. Bōs is a borrowing from Sabellic, having superceded the native **vōs or **ūs, ultimately from Proto-Italic *gʷōs, which is descended from Proto-Indo-European *gʷṓws. So güey ultimately represents a de-shifting of /b/ back to /gw/.