The Collaborative International Dictionary of English v.0.48:

Bedsore \Bed"sore`\, n. (Med.)
   A sore on the back or hips caused by lying for a long time in
   bed.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bedsore
    n 1: a chronic ulcer of the skin caused by prolonged pressure on
         it (as in bedridden patients) [syn: bedsore, pressure
         sore, decubitus ulcer]