vassal (n.)

early 14c. (c. 1200 as a surname) "tenant who pledges fealty to a lord," from Old French vassal "subject, subordinate, servant" (12c.), from Medieval Latin vassallus "manservant, domestic, retainer," extended from vassus "servant," from Old Celtic *wasso- "young man, squire" (source also of Welsh gwas "youth, servant," Breton goaz "servant, vassal, man," Irish foss "servant"), literally "one who stands under," from PIE root *upo "under." The adjective is recorded from 1580s.