“Whoa!” was a command for livestock to stop moving forward. Nelly (or Nellie) was the stereotypical name for a female horse or mule. “Whoa Nelly!” follows the same conventions as “Hold your horses!”. The phrase was popularized on TV via The Roy Rogers Show (1951–1957). read more
The word "whoa" is the word that American use to halt their horses, literally. When horseback riding or with a wagon, you pull on the reins and then say "whoa". "Nelly" was an old standard mare's name - not really a name because Farmer's didn't often name their work tools, the horse being a work tool, was simply, universally, called Nelly. read more