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Day one is a term stolen by basic women (and men) for their "insta" captions. It references a person that is close friend. The term is really referring to someone that has been there since your day one and therefore should only be used by twins who have a literal day one.
Thats my twin he is my actual day one.
Day One by MTVCribs November 30, 2018
Your ride or die bestie, someone who is down to ride and by your side 24/7. Doesn't ask for anything but your loyalty, but would give you everything they have.
Justin is my day one!
Day one by GoddessLeo December 20, 2021

Day one nigga 

A Nigga who never switched up on you.
-<someone who hasn't changed since you met them>-
He stay on some real shit homie, that's my day one nigga tho
Day one nigga by A Day 1 Nigga June 20, 2016

Day One Nigga 

A person who has been completely down to earth and friendly with you since you first met them, they're your nigga since day one.
Kodi and I go way back, we've been friends since first grade. He's my day one nigga!
Day One Nigga by Krelgore June 21, 2013

day one shit 

a technique someone should understand who has been working at a particular trait for a time period longer than the first day of the job status.
Patterson-"Hey, did you guys know that the
safety wire on the utility pump is all jacked up?"

Twat-"Why were you looking at the safety wire on the utility pump?

Patterson-"We are installing one on another bird and wanted to see how it needs to go on."

Twat-"That's day one shit, and your a Sgt. you should know how to do safety wire on a utility pump!!"
day one shit by Killa-D October 21, 2008

Day One Homie 

A friend that you've known for a long time that you still keep in touch with frequently
He's been my day one homie since the 2nd grade
Day One Homie by Electric_Huricane December 10, 2018