Day One (slang)
Plural: Day Ones
Also spelled: Day 1
What does Day One mean?
Someone you’ve been friends or close with since you’ve met them on day 1.
Example sentence: “I’ve known Ope since second grade, that’s my day one.”
Day One in songs:
“You ain’t my day one and I know you not my friend” – Meek Mill, On The Road.