I don't disagree with you
Means you think the other person is lying, but you’re too lazy to call them out. It’s like a middle finger in disguise.
My boss said, 'I don’t disagree with you,' after I told him the project was doomed.
My mom said, 'I don’t disagree with you,' when I told her I was getting a tattoo.
My friend said, 'I don’t disagree with you,' after I told him he failed his math test.