Okay

"Or do you want a coffee?" He straightens, ready to leap up again. "I can have them sent in."
"Uh," I say, unsure as to whether it's a good business move to ask your potential employer to fetch for you. I decide to go with my thirst. "Okay."
Gary stops. "Scat, if you're going to work for Coca-Cola, you will never use that word again."
"What word? 'Okay'?"
"Okay is the most widely known word in the English language. Number two is Coke. We're trying for top spot. Get it?"

This exchange that I just read takes place on page 139 of Max(x) Barry's novel Syrup. I actually put down the book mid paragraph for this...for those who know my reading habits well (and it occurs to me that may be nobody) I don't really like to do that, ever. What baffles me is this: If okay is the most widely know word in the English language, why does the Firefox spell check give it the squiggly red underline of you're-wrongness? Frankly, it's starting to bother me, as I use the word quite often.