That is, who first used the word equilibrium (or equivilent) outside thermodynamics?
This question I thought was interesting?
I think Adam Smith made mentions of equilibrium.
Leon Walras was the first “modern mathematical” economist who wanted to construct economic thought along the lines of physics and differential calculus.
Since equilibrium means so many things for economists (even for the Austrians, who have 3-4 equilibriums), it would be very hard to find the first person who ever thought of an equilibrium concept.