In Pop Culture: A phrase adopted by dissidents who've chosen a
non-violent approach by using truth to oppose perceived acts of oppression by the government.
Note: The phrase is sometimes misappropriated by some groups with irrational fears.
Historically: The best argument so far is that the phrase spoken using these exact words is attributed to
pamphlet called "Speak Truth to Power: a
Quaker Search for an Alternative to Violence", published in 1955. Older phrases similar in nature with practicality the same meaning dated back much earlier.