What’s a “thunk”?

I remember when everyone started talking about using “thunks” in Redux to execute code without blocking the UI. And I remember getting tripped up on the word “thunk”. What’s a “thunk”? Why are we calling it that? The term “thunk” goes all the way back to ALGOL 60! It’s a play on “think”, because the ALGOL compiler needed to “think” about what sort of subroutines to generate. ALGOL allows passing expressions as arguments to subroutines, not just constants.