Cotrgave’s French and English Dictionary [Google Books]:
arriere-fief: m. A mesne fief; a fief that is held of, or depends on another, or higher fief.
soubs-fiefver: C’est bailler en arriere-fief partie de son fief. Ragueau.
Fief: m. A Fief. A (Knights) fee, a Mannor, or inheritance held by homage, and fealty; and given at the first, in trust, and upon promise of assistance, or service in the wars : (A learned Frenchman defines it, L’heritage tenu à foy & hommage, baillé à aucun pour la fiance qu’on a eue en luy; Another, La terre concedée à cause de confiance, ou foy promise par le preneur d’icelle, d’assister son Seigneur en guerre: which both together make good my definition ; ) Also, a Tenure, or Estate in fief, or in fee. This word was first heard of, after the conquest of Gallia by the Francs (or ancient French-men) when their Soveraign Princes, reserving some land for their own Domains, distributed the rest (by whole Countreys, or large territories) among their Captains, and principal followers, on condition, that they should hold of them, and aid them in their wars; in which distribution respect was also had of, and provision made for, the inferior French Souldiers (whereof the more, or fewer those Captains had under them, the greater, or less were their portions) whereupon the Captains, having (as formerly their Princes) reserved somewhat for their particular demains, they divided the best part of the rest among them, to be held of themselves by the same Tenure, on on the same condition, that they held the whole of the King: (Hence came the Arriere fiefs:) the other part they shared among the natural inhabitants of the country, on much baser conditions (expressed in the word Cens :) In those times all Fiefs were determined by the death of the Feoffces (?) and revokable at the will of the Feoffer, but not long after they became )(as the most of the are now) patrimonial, or hereditary.
[My transcription from facsimile of pages in Google Books - Swany]