The story
The earliest attributable use was by Lady Newdigate in a letter written in 1789 (and published in Newdigate-Newdegate Cheverels in 1898): Many happy returns of ye day to us my Dr Love.
Etymology from Wiktionary, CC BY-SA 4.0.
many happy returns
Today it means: A greeting, usually for birthdays, in reference to the passing year. Happy birthday! Full definition →