fly on the wall—an unperceived observer who is able to overhear discussions, etc.:
- “What did you wheedle out of Maggie?” “Well … she was no fly on the wall, but there have to be letters, documents, papers to be typed.”
let that fly stick to the wall—(Scot.) say nothing more on that subject:
- We’ve got in our midst a newspaper; some people call it well, a very peculiar newspaper. But we’ll let that fly stick to the wall.