keep a tight rein on someone/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.

COMMON If you keep a tight rein on someone or something or keep them on a tight rein, you control them firmly and carefully. Note: The reference in these expressions is to a rider using the reins to control a horse. It is said that he kept a very tight rein on his daughters, and that he was a very strict father. The recession has forced people to keep a very tight rein on their finances when on holiday. Her parents had kept her on a tight rein.