1. A male who is not an infant, i.e., a child, young man, or man, depending on context. Can be used either literally or sarcastically/disparagingly. You're a big boy, and you need to learn to clean up after yourself. Sean is a big boy, he can take care of himself.

2. A large and/or formidable person or thing. You're a big boy, but that doesn't mean I won't fight you! Let's take this big boy out for a spin on the open road.

3. slang A term of affection for a man, used by a person who finds that man sexually attractive. Bye big boy, I hope to see you again really soon.

4. The most prominent, powerful, or influential people in a group, organization, or area of business. Usually plural. You're a partner in the firm, so you're going to be working with the big boys now. I'm just a local councilor, I'm not exactly among the big boys of politics.

5. adjective Of, pertaining to, or belonging to a male who is not an infant, with specific age being variable and dependent on context. Often hyphenated. You're growing up so fast, you are already old enough to play on the big-boy swings!