look up any word, like smh:
 
3.
When someone does something so extraordinary.
Timmy draws out his lopsided penis and his mother catches him. Timmy's mom said, "Whoa nelly!"
by LiKWiD July 01, 2003
 
1.
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. When you wanted to stop a (female) horse, you'd say, of course, "Whoa Nelly".

In modern English it's used to refer to anything (i.e. a car, a bike, a situation) or a person, who has become out of control and gone on ahead and done something unexpected - and then it's said after the fact to express shock, awe, or merely surprise, at the unexpected behavior.
"She quit her banking job of 10 years to join an improv comedy troup."
"Whoa Nelly!!...what was she smoking when she did that?"

David and Sarah broke up because he came home smelling like perfume.

The student learning to drive popped the clutch too soon and the car bucked forward, then the driving instructor said, "Whoa Nelly! Be sure to let the clutch out slowly next time!"
by espressowhip July 27, 2010
 
2.
Sexual act.
When you're having sex and you're about to achieve a climax, you reach for your cowboy hat with one hand and put it on then with the other hand you pull off your belt and wrap it around her throat and hold it like the reigns of a horse.
As you climax you pull the reigns back and hold onto your hat with your right hand and yell 'WHOA NELLY!' at the top of your lungs.

+50 points if her name is actually Nelly.
by YourLurchness August 04, 2014