Someone who agrees with the boss no matter what the boss says — not because they think the boss is right, but because agreeing is safe and useful for them. The word is an insult, and everyone who uses it knows that. Calling a colleague a yes-man says two things at once: that he has no opinions of his own that he will speak out loud, and that he is quietly working for his own advantage. You use it about someone under a powerful person — a manager, a president, a company owner — and it often comes in the plural when someone is describing a leader who has hollowed out the people around him: 'he has surrounded himself with yes-men.' That is the most common shape you will meet it in. Note the plural: yes-men, not yes-mans, because 'man' inside it still behaves like the ordinary word man. It is written with a hyphen (yes-man), sometimes as one word or two in casual writing. It sounds a bit dated and a bit male — it comes from American business talk of the 1910s and 20s and still smells faintly of offices full of men in suits. For a woman English has no neat matching word; people say 'yes-woman', which sounds awkward, or reach for something else like 'a rubber stamp' or 'she just tells him what he wants to hear'.