Source : Webster's Revised Unabridged Dictionary (1913)

Orthopnoea \Or`thop*n[oe]"a\, Orthopny \Or*thop"ny\, n. [L.
   orthopnoea, Gr. ?; 'orqo`sstraight, right + pnei^n to
   breathe: cf. F. orthopn['e]e.] (Med.)
   Specifically, a morbid condition in which respiration can be
   performed only in an erect posture; by extension, any
   difficulty of breathing.