From Longman Dictionary of Contemporary Englishbe a fly on the wallbe a fly on the wallto be able to watch what happens without other people knowing that you are there I wish I’d been a fly on the wall during that conversation. → fly-on-the-wall → fly
Examples from the Corpus
be a fly on the wall• I wished I could be a fly on the wall.• I wish I'd been a fly on the wall during that conversation.• You should be a fly on the wall and hear him sing your praises.• Oh, wouldn't I like to be a fly on the wall when you tell her the latest! 