Thin Edge Of The Wedge Meaning

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

Example: 1877 The Analyst, p. 381
  But the great difficulty was the getting in the thin edge of the wedge; when once they had anything to do with price they must apply it to everything.
1903 Albert Shaw The American Monthly Review of Reviews, p. 196
  But at last the thin edge of the wedge of reform had been pushed in.
1996, Dale Belman et al., Public Sector Employment in a Time of Transition, p. 247
  the thin edge of the wedge was further widened by the fact that a socialist party [...] broke precedent [...].