This means that this person is intentionally ignoring somebody or treating someone in an unfriendly way.

It means to make someone feel unwelcome; to insult someone by not treating them as being important.

A story I was told is that it originally meant to serve a guest cold leftovers instead of a hot meal of the best food you have to offer, to show them that you didn't really want them to stay. I doubt that it's true--it sounds like "folk etymology"--but that's one explanation.