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
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A sexual act involving two participants. In this act, the man* is having sex with his partner from behind (i.e. doggy style) while he/she is on all fours. During the final stages prior to ejaculation, the man will commence to wrap his partner's or his own underwear** around his partner's head while grabbing both ends in one hand, rise to a crouched position, and begin jack hammering while forcefully pulling back like a cowboy riding a galloping horse. Just prior to climax, he will yell "whoa nelly", while pulling back one last time.

Variant: At the end, if having vaginal sex, perform a surprise anal insertion.

*Man is a general neutral term to describe the dominant partner in this scenario, it can be a heterosexual or homosexual man, a strap wielding lesbian, or a dominatrix pegging her bitch-boy.
**In a punishment/hate-fuck scenario, heavily soiled underwear are recommended.
I was riding your wife from behind last night, and just before I busted a nut I wrapped my shorts around her head and gave her a "whoa nelly" as I rammed it in her starfish.
by HolyHammer October 12, 2016
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Timmy draws out his lopsided penis and his mother catches him. Timmy's mom said, "Whoa nelly!"
by LiKWiD July 2, 2003
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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 3, 2014
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When a male is unexpectedly outed as a Brony.
Nelson's parents learned he was going to be featured in the new documentary 'Bronies' and said, "Whoa, Nelly! Clop clop right out of our basement and get your own apartment."
by AddressOK December 23, 2012
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when u jerkoff ur penis to a sexual image that u saw,a porno,or something that sexually stimulates u
he was peeking at suzy take her bra off and began whoa nellying
by junior johnson July 15, 2006
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