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]

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