The phrase seems to be U.S. in origin, however, it's definition appears to depend on which part of the country uses it.
As a child I spent much time with my grandfather and other relatives in the Southern U.S. Usually referred to simply as the 'South' but that encompasses a large geographic area, primarily in the south-east but extending west into Texas as well.
During those visits I clearly remember relatives that still made their own high-powered liquor… commonly called "white lightning". The phrase was often used in relation to that product. An Uncle taking a long pull on a bottle of the stuff would try to catch his breath and then in a whispering voice say "That some of the best who-shot-john I've ever had!"