The Collaborative International Dictionary of English v.0.48:

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