Completely meaningless phrase which has gained inexplicable global popularity. It's an incomplete thought: Back in what day? It's a redundant way of saying "Once, I..." or "I used to..." and adds words without adding any extra meaning. Similar to the equally pointless "at the end of the day..." popularised by English soccer stars.
Back in the day, I used to be younger.

I was a child once, back in the day.
by hunthill August 18, 2006
Referring to a time period, at the very least, 2 years ago.
Guy: Remember that baby you had back in the day?
Girl: You bet I do. That was like 2 years ago.
by T_Thisse May 4, 2011
Regarding a time before you mattered. A time that you were not a part of. You must have at least 5 plus years on to even say this phrase.
Back in the day I didn't even work here because I don't matter and should never say back in the day.
by GPJ4TH October 17, 2017
Something that Gen Z'ers will most likely say when they're over 60 years old.
Elderly Gen Z'er: Why back in my day... We had memes, social media, Justin Bieber, bottle flipping, dabbing and internet challenges.

Grandson: Honestly, your generation sounds like it sucked ass horribly.
by CelticEagle February 19, 2019
The way an old man tells you to stop whining about shit that people back in their day did differently
Old Man: Back in my day, we didnt have toilet paper, we just wiped our asses with leaves

Young Man: Holy Shit, that must've sucked ass.
by GrapeRape69 October 16, 2014
A phrase typically used by old people to criticize younger people or to justify themselves.
Kids today are always on their phones, but back in my day we were only allowed to play with a wooden stick!
by kho25 March 6, 2022
The prefix of a phrase uttered by elders and adults describing what it was like to do something in their own past.
Old Man Ulysses: Back in my day, we would look up Webster's Dictionary instead of Urban Dictionary. No biased information there!
Pizza Guy: WAT 0_0
Wordington-Chan: Gee, I didn't know that. Thanks for the enlightment~! ^w^
Old Man Ulysses: You're welcome, lass. Anyways, back in my day, instead of ordering pizza, we made our own on our farms.
Pizza Guy: WAT ):<
Old Man Ulysses: Back in my day...
Pizza Guy: WATEVER (Yeah, I know, nobody used emoticons in their own dialogues.)
by Mr. Wordington the Tenth October 31, 2018