keep a tight rein on somebody/something
keep a tight rein on (someone or something)
To maintain strict or tight control over someone or something; to not allow someone or something very much independence or autonomy. Ever since George nearly lost his life savings in a drunken poker match, his husband has started keeping a tight rein on him. The boss has kept a tight rein on her assistant since she hired her. We try to keep a tight rein on our expenditures, but that's not always possible.
keep a tight rein on someone or somethingand keep a close rein on someone or something
Fig. to watch and control someone or something diligently. (Alludes to controlling a horse by a tight grip on the reins.) The office manager kept a tight rein on the staff. Mary keeps a close rein on her children.
keep a tight rein onexercise strict control over; allow little freedom to.
keep a tight ˈrein on somebody/something(also keep somebody/something on a tight ˈleash) control somebody/something very carefully; give somebody/something very little freedom: The company must keep a tight rein on spending. ♢ She keeps her children on a tight leash to make sure they don’t get into trouble. OPPOSITE: give/allow somebody/something free/full rein
A leash is used to hold and control a dog, and a rein is used to control a horse.