keep a tight rein on someone/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.
See also: keep, on, rein, tight
Farlex Dictionary of Idioms.

keep a tight rein on someone/something

or

keep someone/something on a tight rein

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.
See also: keep, on, rein, someone, something, tight
Collins COBUILD Idioms Dictionary, 3rd ed.
See also:
  • better of
  • be/have done with somebody/something
  • be in line with (someone or something)
  • (someone or something) promises well
  • bird has flown, the
  • begin with
  • begin with (someone or something)
  • beware of
  • beware of (someone or something)
  • be rough on (someone or something)