The Collaborative International Dictionary of English v.0.48:

Fatten \Fat"ten\, v. t. [imp. & p. p. Fattened; p. pr. & vb.
   n. Fattening.] [See Fat, v. t.]
   1. To make fat; to feed for slaughter; to make fleshy or
      plump with fat; to fill full; to fat.
      [1913 Webster]

   2. To make fertile and fruitful; to enrich; as, to fatten
      land; to fatten fields with blood. --Dryden.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fatten \Fat"ten\, v. i.
   To grow fat or corpulent; to grow plump, thick, or fleshy; to
   be pampered.
   [1913 Webster]

         And villains fatten with the brave man's labor.
                                                  --Otway.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fatten
    v 1: make fat or plump; "We will plump out that poor starving
         child" [syn: fatten, fat, flesh out, fill out,
         plump, plump out, fatten out, fatten up]

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "fatten":
   acculturate, add to, advance, aggrandize, aliment, ameliorate,
   amend, amplify, augment, batten, become overweight, better, bloom,
   blossom, boom, boost, breast-feed, breed, bring forward, broaden,
   build, build up, civilize, culture, dress, dry-nurse, edify,
   educate, elevate, emend, engross, enhance, enlarge, enlighten,
   enrich, exalt, expand, extend, farm, fat, fatten up, favor,
   fecundify, feed, fertilize, fill out, flourish, flower, force-feed,
   forward, foster, fructify, gain weight, gather flesh, go straight,
   grow, grow fat, grow thick, hatch, hike, hike up, impregnate,
   improve, improve upon, incrassate, increase, inflate, inseminate,
   inspissate, jack up, jump up, keep, lactate, lard, lengthen, lift,
   make an improvement, manure, maximize, meliorate, mend, nourish,
   nurse, nurture, nutrify, parlay, pinguefy, plump, prolificate,
   promote, put on weight, put up, pyramid, raise, ranch, rear,
   refine upon, reform, run, socialize, straighten out, stuff, suckle,
   thick, thicken, thrive, transfigure, transform, up, upgrade,
   uplift, wet-nurse