ear hustler
Someone who eavesdrops, usually in a place where people are talking, and uses the secrets to make others look stupid or get an easy win.
He listened to the boss’s plan and told me before the meeting. I looked like a genius, and he looked like a backstabber.
She heard the teacher’s password and used it to log in. The teacher was confused, and I was amused.
He eavesdropped on the lunch plans and showed up at every table. Everyone was confused, and he was full.