The Collaborative International Dictionary of English v.0.48:

Luthern \Lu"thern\, n. [F. lucarne a dormer, dormer window,
   garret window, L. lucerna lamp, fr. lucere to be light or
   clear, fr. lux light. See Light, n., and cf. Lucarne.]
   (Arch.)
   A dormer window. See Dormer.
   [1913 Webster]