The interjection “whoa” – defined in the Oxford English Dictionary as: a command to a horse to stop or stand still” or “a general interjection expressing surprise, delight, etc.” has been in use since the early 19th century.
Consider, for example, the use of the word as an intransitive verb in an 1838 issue of New Sporting Magazine: “He..climbed up the fence, ‘whoaing’ and crying to his horse to ‘stand still’.”
There is some evidence that the word has existed since long before that. One etymological dictionary, for example, dates the word to the 1620s; defining it as “a cry to call attention from a distance, a variant of who.”
But in the age of the internet, a funny thing has started happening:
Whoa. W. H. O. A. has more and more frequently come to be spelled as ‘woah’, as if the ‘h’ is precariously trying to escape from the whole messy situation.
In 2013, Slate wrote a whole piece on the gaining popularity of the wrong / new spelling: “All things considered, it’s been a banner year for “whoa,” no matter how you prefer to spell it,” they write.
And, as Mashable points out, the ACLU and Merriam-Webster dictionary recently sorted the whole thing on Twitter:
“We don’t include [woah] as a variant,” Merriam-Webster wrote in response to a query from the ACLU, “but we’re pretty sure you still have the right to say it.”
That is, after all, what it means for English to be a living language.