An “old thing” refers to relationship. When someone uses “old thing,” it’s typically because they have nostalgic sentiment for it.

When someone says they want “the old thing back,” (which is the context it is used in majority of the time), it means that they want things to go back to the way they were before. They miss how things were between them and their ex. It is usually used when substantial time has passed between the present and the relationship they had with their ex, OR there has been a long period of time since things went south in their relationship with their ex.
Devin: “Yo I saw Aline earlier at McDonalds.”

Omari: “Forreal? Damn I miss that girl. I want that old thing back...”
by DeltaIndiaCharlieKilo January 20, 2018
An "Old thing" an Ex boyfriend or Ex Girlfriend Whom You've Since Elivated from
Chris: "This the shorty I was telling you about"

Ray Ray: "Damn son, You Know that's my Old Thing right"
by SkinComplexionHenny February 2, 2014
.Stick it in that old thing is a phrase used by the YouTubers Woody And Kleiny
Hey my Woody lad stick it in that old thing
?Oh what? that old thing
.it's a little bit too easy
by Manager Darryl November 18, 2020
Jennie went to Amish Country in search of Old Timey Things and came home with a truck full of quilts.
by crazyhrlady July 27, 2009
Use these 3 words as a way to say something about parents/adults with alcohol.
My parents are going out to go do their old people thing.(wine tasting).
by Im_A_Kool_Kid777 June 26, 2022