emeritus (adj.)
c.1600, from Latin emeritus "veteran soldier who has served his time," literally "that has finished work, past service," past participle of emerere "serve out, complete one's service," from ex- "out" (see ex-) + merere "to serve, earn," from PIE *(s)mer- "to get a share of something" (see merit (n.)). First used of retired professors 1794 in American English.