1 definition by stair lift
on september 18th anything someone does to you u have to do back for e.g. if someone steals some of your food you steal some of theirs.
friend: steals one of your chocolate bars
friend 2: hey its backsies day steal one of his
you: omg yes its september 18th
friend 2: hey its backsies day steal one of his
you: omg yes its september 18th
by stair lift September 18, 2021