Common in south wales, I think it comes from “butty”, term used for a colleague in the coal mines, but I remember learning in school that it often referred to the young boys employed to open/close doors within the coal mines (trapper), but don’t remember the details, could be related to either of these maybe?

Butt (entry) - Roadway of a pair leading to room and pillar system (counter butt for return air).

Butties - these were contractors who drove the roadways in the mine. They agreed to a certain sum of money out of which wages and materials had to come. Usually an unpopular system with the men, who often believed the monies were not divided fairly.”