If you say you won't be kept down by the man, you are saying that you won't let the man subjugate you. To subjugate is to repress someone, or to make them subservient to you.
In subjugate you see the word subject. In this word, it's not talking about the subject of a sentence, rather it's talking about the kind of subjects that Kings have serving them. Subjugate is to reduce someone's status to that of a subject. It's a royal demotion, often brought by force or intimidation.