From Wiktionary, the free dictionary
frog in one's pocket
- (humorous) The supposed reason for a person using the pronoun we when really speaking only about themselves.
2016, David Hricik, Law School Basics: A Preview of Law School and Legal Reasoning:[…] the interrogating lawyer said: "Who is this 'we'? Do you have a frog in your pocket?"
2018, Ross Klavan, Tim O'Mara, Charles Salzberg, Three Strikes: 3 Authors, 3 Novellas:“We got a problem.” We? I'm thinking. You got a frog in your pocket or something?
