How do I turn off the red squiggly lines?

How do I turn off the red squiggly lines?

I need to turn off the red squiggly lines that appear before "misspelled" words by default when I'm composing an email.  I use the word "playwriting" very frequently, which apparently nobody thinks is a word.  So I always ignore the red squiggly lines.  Then, when I *did* spell it wrong just now -- in the subject line of an email to someone I was trying to impress -- it went unnoticed, because I'm so used to ignoring the lines.

So I just really just want to turn them off.  So many of them appear so unnecessarily, and sometimes I even switch to plain text just because the lines aren't quite as ostentatious.  If I can be pointed to the way to get rid of them, I'd appreciate it.