Source : Webster's Revised Unabridged Dictionary (1913)

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.