Green’s Dictionary of Slang

(US) demoted, deprived of a position of power or authority.

1931		Irwin Amer. Tramp and Und. Sl. 61: Dehorn. – Anything inimical to I.W.W. belief or welfare, or anything which seemed likely to weaken the movement or its workers.
1945		F.H. Hubbard Railroad Avenue 340: Dehorned – Demoted or discharged.