I don't use this phrase myself. As far as I can recall, the only time someone said it to me, they were lying (I knew what had actually happened), and obviously not going to tell the truth at all. I now associate this with someone being untruthful. And h used it tonight when I was asking him about something. He also has form for lying.
If you use this phrase, under what circumstances do you use it? Avoiding the whole truth? Fudging? Annoyed with someone who doesn't take your word as law? Or something else.
And how has someone used the phrase in conversation with you?
I'd like to think I'm overthinking it, but there's something dismissive about this phrase imo.