your, his, etc. word is law

(one's) word is law

One has the ultimate authority; you must do what one says. You better do it, or you know what'll happen. Mom's word is law. I don't care what your supervisor said. I'm the boss here, so my word is law.
See also: law, word

your, his, etc. ˌword is ˈlaw

used to say that somebody has complete power and control: Their father is very old-fashioned. His word is law in their house.
See also: law, word