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late 13c., from Old French fieu, fief "fief, possession, holding, domain; feudal duties, payment," from Medieval Latin feodum "land or other property whose use is granted in return for service," widely said to be from Frankish *fehu-od "payment-estate," or a similar Germanic compound, in which the first element is cognate with Old English feoh "money, movable property, cattle" (also German Vieh "cattle," Gothic faihu "money, fortune"), from PIE *peku- "cattle" (cf. Sanskrit pasu, Lithuanian pekus "cattle;" Latin pecu "cattle," pecunia "money, property"); second element similar to Old English ead "wealth."
OED rejects this, and suggests a simple adaptation of Germanic fehu, leaving the Medieval Latin -d- unexplained. Sense of "payment for services" first recorded late 14c. Fee-simple is "absolute ownership," as opposed to fee-tail "entailed ownership," inheritance limited to some particular class of heirs (second element from Old French taillir "to cut, to limit").