What is Lex Iniusta Non Est Lex?

Legal Definition
Lex iniqua lex non est (Latin: An unjust law is no law at all), is a standard legal maxim.

Originating with St. Augustine, the motto was used by St. Thomas Aquinas and quoted by Martin Luther King Jr. during the Civil Rights Movement to describe racial segregation and discrimination against Black people.

This view is strongly associated with natural law theorists, including John Finnis and Lon Fuller.
-- Wikipedia