thin edge of the wedge
A minor change that begins a major development, especially an undesirable one. For example, First they asked me to postpone my vacation for a week, and then for a month; it's the thin edge of the wedge and pretty soon it'll be a year. This term alludes to the narrow wedge inserted into a log for splitting wood. [Mid-1800s]
Words nearby thin edge of the wedge
thimerosal,
thimphu,
thin,
thin as a rail,
thin client,
thin edge of the wedge,
thin film,
thin on top,
thin register,
thin section,
thin-film