spare the rod and spoil the child - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Discipline is necessary for good upbringing, as in She lets Richard get away with anything—spare the rod, you know. This adage appears in the Bible (Proverbs 13:24) and made its way into practically every proverb collection. It originally referred to corporal punishment. It is still quoted, often in shortened form, and today does not necessarily mean physical discipline.