The manager treats his department as his personal fiefdom.

fiefdom 是由名词 fief（封建领主的封地）加上后缀 -dom（表示"领域、状态"）构成的名词。fiefdom 原指封建领主的领地，引申为"个人完全掌控的领域或势力范围"，常带有贬义，暗示独断专行的管理方式。