A derogatory name for Coors beer, or Coors beer that has gone bad in the can or bottle. The term was popular in the 1970's, when Coors was difficult to obtain in various parts of the USA. Adventurous beer enthusiasts would drive out to Colorado from east of the Mississippi and bring it back unrefrigerated.
Even if re-refrigerated, it was still skunked, considered swill,and difficult to drink.
"That stuff was in Tony's trunk for three weeks; I'm not drinking Rocky Mountain Gopher Piss."