1533, from L.
secessionem (nom.
secessio), from pp. stem of
secedere "secede," from
se- "apart" (see
secret) +
cedere "to go" (see
cede). Originally in a Roman historical context, "temporary migration of plebeians from the city to compel patricians to address their grievances;" modern use in ref. to religious or political unions dates from 1660.
Secede is attested from 1702, originally "to leave one's companions;" sense of "to withdraw from a political or religious alliance of union" is recorded from 1755.
Secessionist first recorded 1860 in U.S. context (short form
secesh is attested from 1861); the earlier noun had been
seceder, but this had religious overtones, especially in ref. to Scottish Church history.