a Pertaining to engines, vehicle movement etc. moving in a direction opposite to the usual direction. (defdate|from 19th c.)
n The tails side of a coin, or the side of a medal or badge that is opposite the obverse. (defdate|from 17th c.)
n The side of something facing away from a viewer, or from what is considered the front; the other side. (defdate|from 18th c.)
v (chemistry) To change the direction of a reaction such that the products become the reactants and vice-versa.