The story
Spoken by Neil Armstrong when he first stepped onto the surface of the Moon. Armstrong intended to say "one small step for a man", but misspoke.http://www.snopes.com/quotes/onesmall.asp
Etymology from Wiktionary, CC BY-SA 4.0.
one small step for man, one giant leap for mankind
Today it means: (used imitatively) A cliché used to exaggerate an accomplishment or milestone. Full definition →