I don't disagree with you
You’re pretending to agree, but you think the other person is a complete idiot and you’re going to make them feel it later.
My boss said, 'I don’t disagree with you,' when I told him the whole plan was stupid.
My brother said, 'I don’t disagree with you,' when I told him he lost the bet.
My friend said, 'I don’t disagree with you,' when I told him the party was a disaster.