mid-15c. (c.1200 as a surname) "tenant who pledges fealty to a lord," from Old French vassal, from Medieval Latin vassallus "manservant, domestic, retainer," from vassus "servant," from Old Celtic *wasso- "young man, squire" (cf. Welsh gwas "youth, servant," Breton goaz "servant, vassal, man," Irish foss "servant"). The adjective is recorded from 1590s.
Under feudalism, a subordinate who placed himself in service to a lord in return for the lord's protection.