C.B. from Malibu, California, wrote, "How do we stop people from spelling 'whoa' as 'woah'? Please tell me when this misspelling started. Even professional websites misspell it."
Here's a way to remember the proper spelling: Who and ho are two origins that are often cited for whoa. For example, Dictionary.com states that ho came first as a Middle English command to make a horse stop and then evolved sometime around 1620 into whoa. To remember how to spell whoa, remember that the original word—ho, something you'd say to a horse—stays intact in the middle.
Also, in 1616 in the play "The Winter's Tale," William Shakespeare had a character known only as Shepherd call out "Whoa-ho-hoa!" Say it in your mind: whoa-ho-hoa. Say it loudly like Santa Claus: whoa-ho-hoa! The extra hos after the whoa should also help you remember to keep the ho in the middle.
2/27/2018: Here's a fascinating follow-up: "Woah" is more common and accepted in Britain than it is in the US, according to Lynne Murphy's research on the Separated by a Common Language blog.
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