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.

Sexual act.

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.

When a male is unexpectedly outed as a Brony.

when u jerkoff ur penis to a sexual image that u saw,a porno,or something that sexually stimulates u