The Mickey Finn was originally tied by Quebec fly tier Charles Langevin sometime in the 19th century. But it did not go by that name until John Alden Knight popularized the pattern. Knight and his fishing partner originally named the pattern The Assassin after hauling in a large number of fish one day. But a newspaper columnist named the pattern Mickey Finn and then Knight went on to give it even more publicity.
Way back in 1903 a bartender in Chicago named Mickey Finn was using knockout drops in customer’s drinks to incapacitate them and rob them. This became a general name of lacing drinks with drugs. This is where the name comes from. Anyway, the name stuck and it is still a very popular classic streamer pattern.
Hammer Creek Fishing recently produced a video on how to tie this classic streamer.