1
general::
Assert something positively and often arrogantly, state something dogmatically. For example, Dad laid down the law about locking up the house. This colloquial expression, first recorded in 1762, uses LAY DOWN in the sense of def. 2.
American Heritage Idioms
2
general::
Phrase(s): lay down the law (to someone) (about something)
Fig. to scold someone; to make something very clear to someone in a very stern manner. • Wow, was she mad at Ed. She really laid down the law about drinking to him. • She laid down the law to Ed. • She laid down the law about drinking.
McGrawhill's American Idioms And Phrasal Verbs