Gayle King is catching some flak online after casually using a gay slur on live TV ... throwing it out in the middle of a joke she was quoting.

The TV host was interviewing Matteo Lane -- a well-known author and comedian -- on "CBS Mornings" about his new book "Your Pasta Sucks: A Cookbook" ... when she quoted one of Lane's jokes back at him.

King says, "Can we talk about the stand-up? Can I just say one joke? I hope I don't get in trouble. You do a riff about white women who approached you, and they said something about cooking, and you said, 'What in the f****try are you talking about?'"

Lane busts up laughing at King's rendition of the joke and tells her he absolutely loves her ... though her cohosts seem to cringe a little.

So, Lane clearly wasn't offended by the use of the word ... and, King hasn't apologized -- so, it seems everyone's walking away from this moment without any outrage. At least, everyone directly involved in the moment ain't outraged.

While it was a lighthearted moment on the show, some online didn't appreciate Gayle's words much ... calling the host out for using the slur even though she was quoting a joke.

We've reached out to King for comment ... so far, no word back.