c. 1300, of lords, "entitled to feudal allegiance and service," from Anglo-French lige (late 13c.), Old French lige "liege-lord," noun use of an adjective meaning "free, giving or receiving fidelity" (corresponding to Medieval Latin ligius, legius), a word of uncertain origin. Perhaps from Late Latin laeticus "cultivated by serfs," from laetus "serf, semi-free colonist," which probably is from Proto-Germanic *lethigaz "freed" (source also of Old English læt "half-freedman, serf;" Old High German laz, Old Frisian lethar "freedman;" Middle Dutch ledich "idle, unemployed"), from extended form of PIE root *lē- "to let go, slacken." Or the Middle English word might be directly from Old High German leidig "free," on the notion of "free from obligation to service except as vassal to one lord," but this reverses the notion contained in the word.
From late 14c. of vassals, "bound to render feudal allegiance and service." The dual sense of the adjective reflects the reciprocal relationship it describes: protection in exchange for service. Hence, liege-man "a vassal sworn to the service and support of a lord, who in turn is obliged to protect him" (mid-14c.).