The very beginning.
I've been with the project since day one.
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(US, slang) A close, longtime friend, one who has been around since the beginning.
2017 March 17, G-Eazy and Kehlani, "Good Life", from the soundtrack for The Fate of the Furious:
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At the very beginning; when something is entirely new.
to buy something day one
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Add your own interpretation of "day one".
Regional slang from around the English-speaking world — British, Australian, Irish, Caribbean, Nigerian, Filipino, AAVE, and the hyphenated-English dialects that make the internet sound local.
See all Regional & Other slang on Slangora.