Yours, Etc.: Origins and Uses of 8 Sign-Offs
1780, in reference to members of U.S. Congress, and it first appears in a piece of abuse (written by a Loyalist):
Ye coxcomb Congressmen, declaimers keen,Congresswoman attested from 1918 (Jeannette Rankin (1880-1973) was the first).
Brisk puppets of the Philadelphia scene ...