The Collaborative International Dictionary of English v.0.48:

Sofa \So"fa\, n.; pl. Sofas. [Ar. soffah, from saffa to
   dispose in order: cf. F. sofa, It. sof[`a].]
   A long seat, usually with a cushioned bottom, back, and ends;
   -- much used as a comfortable piece of furniture.
   [1913 Webster]

         Let fall the curtains, wheel the sofa round. --Cowper.
   [1913 Webster]

   Sofa bed, a sofa so contrived that it may be extended to
      form a bed; -- called also sofa bedstead.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sofa bed
    n 1: a sofa that can be converted into a bed [syn:
         convertible, sofa bed]