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]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "waif":
   Arab, beach bum, beachcomber, beggar, bo, bum, bummer, castaway,
   castoff, derelict, discard, dogie, flotsam, flotsam and jetsam,
   foundling, gamin, gamine, guttersnipe, hobo, homeless waif, idler,
   jetsam, junk, lagan, landloper, lazzarone, loafer, losel, mudlark,
   orphan, piker, ragamuffin, ragman, ragpicker, refuse, reject,
   rounder, rubbish, ski bum, stiff, stray, street Arab,
   street urchin, sundowner, surf bum, swagman, swagsman,
   tatterdemalion, tennis bum, tramp, trash, turnpiker, urchin, vag,
   vagabond, vagrant, waifs and strays, wastrel



Bouvier's Law Dictionary, Revised 6th Ed (1856):

WAIFS. Stolen goods waived or scattered by a thief in his flight in order to
effect his escape.
     2. Such goods by the English common law belong to the king. 1 Bl. Com.
296; 5 Co. 109; Cro. Eliz. 694. This prerogative has never been adopted here
against the true owner, and never put in practice against the finder, though
against him there would be better reason for adopting it. 2 Kent, Com. 292.
Vide Com. Dig. h.t.; 1 Bro. Civ. Law, 239, n.