To suggest something to someone; to lead someone to start believing (in) something; to make someone want to start doing something. (Used of suggestions, ideas, or beliefs that someone wouldn't have thought of on their own, especially those that are foolish or negative.) Of course we're not going to sell the house! Who put that idea in your head? I can't believe you were caught smoking marijuana. I bet it's that no-good kid Casey who put an idea like that into your head. I heard John putting an idea into his sister's head that she was adopted.