under the impression
Holding the assumption or belief (that something is true). Oh, sorry, I was under the impression that you two were a couple. My mistake. A: "Aren't you planning on retiring soon?" B: "Why would you be under that impression?"
under the impression
Thinking, assuming, or believing something, as in I was under the impression that they were coming today. This idiom often suggests that the idea or belief one had is mistaken. [Mid-1800s]