Noun
the thin end of the wedge
(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.
When reporters say to me I'm only doing this because it's my job... that's the same abdication of moral responsibility at the thin end of the wedge that in its most extreme and horrific version ends up with others being prepared to stand as a concentration camp guard. Ken Livingstone