How can we know? You know this man and your history with him. How do you interpret it?
If my DH said that, I know he would mean it. I, on the other had, will say things when I am mad, that I don't necessarily mean, just because I am hurt and feel threatened.
Is he a guy that gets mad and says things, or one that means every word that comes out of his mouth?

Well, I only say what I mean, but my DH often says things, that he doesn't really mean in an argument. So it could be either way. But

The truth is in a marriage, or other long term relationship, is that sometimes you do get sick of them! But then you love them again. It is a cycle, but does not mean you are through.

There are certain boundaries I feel should NOT be crossed in an argument between my DH and I. His M.O. to be hurtful was to say "I'm leaving." The first time he said it, I replied "Do not threaten something you're not willing to carry thru with." The second time he said it, I handed him a duffel bag. Told him I'd already warned him once, and if cares so little about this relationship to throw down the "I'm leaving" card when things get tough then that's what he needs to do.

He's never said it again.

The bad things about horribly heated arguments sometimes is that they become personal attacks. We've all been guilty of it. I think you need to decide what is and is not OK and be very clear about it. My own opinion -- it's one thing for DH to say "I'm sick of this". But when he says "I'm sick of YOU". At least to me that would be unacceptable.

I had a problem for the first 5 years of the marriage with DH announcing "If your so unhappy then we should just get divorced" or remarks like "If your not happy, why do you even stay married." and the final straw was "I don't want to feel like I am just your meal ticket" It was always him that was unhappy with me because I was refusing to go along with something he wanted to do for his adult children. Took a while for me to realize never once had I said I was unhappy with the marriage and wanted out---always him .

This was his way of turning things around so he would look like the poor guy that was left by the wife again. Push the decision onto the other person and he could look innocent of wanting out.

I picked a day when there was no friction between us and told him I needed to talk. "DH in all the years we have been married I can not think of one time I have said to you that I want out of this marriage when I could not have my way. On the other hand you have repeatedly said you were unhappy and suggested the possibility of splitting. Do not give me an answer now. In 24 hrs. we will sit down and talk again. At that time you will give me your answer if you want out. If you are THAT unhappy I will not stand in your way. If you decide to stay---- Do not EVER say such things to me again."

It could have gone either way but in our case he decided to stay. He said these things because he had gotten away with it in past relationships and no one ever called him on it. This type of bullying stops when it no longer works in their favor.

I am with stepfamily friend on this one. Only you can know whether your SO truly means he's sick of you or is speaking in the heat of the moment. My SDs BM says outrageous, unrepeatable things to her daughters, that never in my wildest nightmares would I have said to my child, and them back to her, then a little while later are all "best friends forever" again.
On the other hand, both my exDH and my DH only tend to say things if they really mean them, me too. You need to judge for yourself.

"Sick of you" could mean a bunch of things, most notably "sick of you NOT being a doormat to the BM, skids and me."

GG has said the most horrible, awful things to me that he wouldn't DARE say to the BEhemoth (BM) b/c she's the "mother of his children" (TM) and he had his uber meddling, attached at the umbilical cord MIL, the Sasquatch constantly monitoring their marriage on a daily basis.

Seems that since I said "well have your boss buy this house (my house) and I'll move out" he's been extremely careful about his speech. It's been three months now that he has not blown up at me, which is a new record since I officially ended my "doormat days."

(knock on wood)