When was the first movie pie fight?

The first movie pie fight was in 1913, for a Keystone Studio comedy.

Mabel Normand tossed a workman’s lemon meringue pie at Ben Turpin to get him to laugh.

He did; Mack Sennett saw it and the pie scene became a favorite bit in Keystone Kop comedies, and in many other comedies as well.