The Collaborative International Dictionary of English v.0.48:

Waif \Waif\, n. [OF. waif, gaif, as adj., lost, unclaimed, chose
   gaive a waif, LL. wayfium, res vaivae; of Scand. origin. See
   Waive.]
   [1913 Webster]
   1. (Eng. Law.) Goods found of which the owner is not known;
      originally, such goods as a pursued thief threw away to
      prevent being apprehended, which belonged to the king
      unless the owner made pursuit of the felon, took him, and
      brought him to justice. --Blackstone.
      [1913 Webster]

   2. Hence, anything found, or without an owner; that which
      comes along, as it were, by chance. "Rolling in his mind
      old waifs of rhyme." --Tennyson.
      [1913 Webster]

   3. A wanderer; a castaway; a stray; a homeless child.
      [1913 Webster]

            A waif
            Desirous to return, and not received. --Cowper.
      [1913 Webster]

WordNet (r) 3.0 (2006):

waif
    n 1: a homeless child especially one forsaken or orphaned;
         "street children beg or steal in order to survive" [syn:
         waif, street child]