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The Collaborative International Dictionary
portuse

Portass \Por"tass\, n. [OF. porte-hors a kind of prayer book, so called from being portable; cf. LL. portiforium.] A breviary; a prayer book. [Written variously portace, portasse, portesse, portise, porthose, portos, portus, portuse, etc.] [Obs.]
--Spenser. Camden.

By God and by this porthors I you swear.
--Chaucer.