thin end of the wedge

• small end of the wedge
• thin edge of the wedge

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the thin end of the wedge

1. (idiomatic) Something that if allowed or accepted to a small degree would lead to systematic encroachment.

   The MP told parliament that legalising cannabis would be the thin end of the wedge.

   • 2018 August 14, April Stroud, Making Sense of Land Law, Bloomsbury Publishing, →ISBN:

     This is known as the 'thin end of the wedge' argument raised by people who don't want to lose the protection of restrictive covenants. The 'thin end of the wedge' means that when you drive a wedge into something, you use the thin end first because it's easier than using the thick end.

• give them an inch and they'll take a mile