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One who always agrees with authority figures, in order to gain favor with them. Stephanie only hired Tim because he's a yes man and won't challenge her authority.
Someone who agrees to everything, seemingly without giving it a thought. Usually a derisive term suggesting that the person is a mindless follower of what/whomever they are agreeing to/with. You can't trust what Chad says, he's just the boss's yes-man. I doubt he has an original thought in his head!