I do not know the origin of the term Jim Dandy - but the 1950's song was inspired by a family dog that my grandmother owned in the early 50's. Lincoln Chase was hiding out at her house from gangsters who were trying to collect on a bad debt. When they broke in with guns in hand - Jim Dandy (a weinie dog) attacked them, giving Lincoln Chase and my grandpa enough time to wake up and kick their ass. Chase wrote the song in his gratitude and often thought that jim Dandy had saved his life.