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
Get the mug
Get a Old Thing mug for your fish Larisa.
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 02, 2014
Get the mug
Get a old thing mug for your father Georges.
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
Get the mug
Get a Old Timey Things mug for your coworker Vivek.