This has always confused me. I've always spelled it "moustache", but my browser's spell checker claims the correct spelling is "mustache". From what I've seen around the Internet, people seem to use both.
What is the proper spelling? Are both acceptable?
This seems like a good one for Ngram.
It looks like in British English moustache has always been the preferred spelling.
In American English, it's been more of a battle with mustache now in the lead.
So here's a combined view. Both are contenders in the English-speaking world, but clear preferences exist on opposite sides of the pond.
(Have to wonder if the peaks and valleys correspond to mustaches going in and out of vogue.)
Both spellings are correct. Which is more standard will depend where you are. Mustache is somewhat more common in American English, and has been since the 1940’s. Moustache is older (being the original French spelling, from which the English word is derived) and still much more common in British English, to the extent that mustache may be seen as an Americanism; but mustache is now gaining some ground in Britain too.
(Sources: Google N-grams viewer for usage, and the OED for etymology.)
Moustache is a variant, possibly on its way out.
But don't be intimidated by the squiggly lines in your browser's text fields. If you know a word is spelled right, go ahead and use it. Most of the time when I can't decide on a spelling it's a variant anyway. Browsers just aren't that smart.
Moustache was definitely the spelling in mind when Movember was coined:
Movember (a portmanteau word from moustache and "November") is an annual, month-long event involving the growing of moustaches during the month of November to raise awareness of prostate cancer and other male cancer and associated charities. The Movember Foundation runs the Movember charity event, housed at Movember.com The goal of Movember is to "change the face of men's health."
The Welsh word for November is Tachwedd, so in Wales Movember is called Tache-wedd - a really pleasing bilingual coinage, well done whoever thought that up!
The proper spelling depends on which English you use. The "mustache" spelling (which just had a wavy, red line underneath, when I typed it) is American spelling. The "moustache" spelling is English spelling.
This is demonstrated in the following link http://dictionary.cambridge.org/dictionary/british/moustache?q=moustache , which says (US usually mustache).
