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. [1913 Webster] By God and by this porthors I you swear. --Chaucer. [1913 Webster]