(out) over (one's) skis

Thinking or acting impetuously or preemptively, before the reality or truth of something is revealed. A: "I can't believe he told you to leave!" B: "Oh, he was out over his skis, as usual. I don't pay much attention to what he says." You have no way of knowing what Shelly's thinking, and you're over your skis if you think you do.