The Collaborative International Dictionary of English v.0.48:

Dormant \Dor"mant\, n. [See Dormant, a.] (Arch.)
   A large beam in the roof of a house upon which portions of
   the other timbers rest or " sleep." --Arch. Pub. Soc. --
   Called also dormant tree, dorman tree, dormond, and
   dormer. --Halliwell. Dormer