Is it farther or further?
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.
in feudal society, one invested with a fief in return for services to an overlord. Some vassals did not have fiefs and lived at their lord's court as his household knights. Certain vassals who held their fiefs directly from the crown were tenants in chief and formed the most important feudal group, the barons. A fief held by tenants of these tenants in chief was called an arriere-fief, and, when the king summoned the whole feudal host, he was said to summon the ban et arriere-ban. There were female vassals as well; their husbands fulfilled their wives' services.