have/keep somebody on a string
have (someone) on a string
To have great influence over someone. If you want to have someone on a string, flattery always helps.
keep (one) on a string
To maintain great influence over one. If you want to keep someone on a string, flattery always helps.
have/keep somebody on a ˈstringmake somebody do what you want because you have control over them: Of course, he’s rich and powerful enough to keep several people on a string.
This idiom refers to a puppet (= a model of a person or an animal) that is controlled with strings attached to parts of its body.