Season 171 - Day 3 of 14

Identify the idiom that means the genuine article, and is often used to distinguish the original or authentic version from a counterfeit or imitation.
The etymology of the phrase is unclear. Some scholars attribute the idiom to a mid-nineteenth century poem called Deil's Hallowe'en.
Others attribute it to a reference to the skills and inventions of a Scottish-American 19th century inventor and engineer.