The story
From whoa! (a command to slow or stop) + Nelly (a common name for a mule or horse). A popular catch-phrase, together with "Whoa, Nellybelle" on the Roy Rogers TV show (1951–1957).
Etymology from Wiktionary, CC BY-SA 4.0.
whoa, Nelly
Today it means: an exclamation of surprise, especially one in response to an unexpected acceleration of speed Full definition →