You're typing along, you make a mistake, and you see the wavy red underline.
You fix the mistake, but you've lost your flow, your idea. To avoid that distraction, you can turn spell check off, then check spelling manually when you're done writing. Here's how.
Click Edit Document > Edit in Word Web App if you haven’t already opened your document for editing.
On the Review tab, click Spelling, and click Set Proofing Language.
In the Language box, click Don’t check spelling, and click OK.
Remember to run spell check. Spelling and grammar errors can seriously undermine what you're trying to say, especially when your boss, your teacher, or that person in HR sees them.