The Collaborative International Dictionary of English v.0.48:

Thin \Thin\, v. t. [imp. & p. p. Thinned; p. pr. & vb. n.
   Thinning.] [Cf. AS. ge[thorn]ynnian.]
   To make thin (in any of the senses of the adjective).
   [1913 Webster]

WordNet (r) 3.0 (2006):

thinning
    n 1: the act of diluting something; "the cutting of whiskey with
         water"; "the thinning of paint with turpentine" [syn:
         cutting, thinning]

Moby Thesaurus II by Grady Ward, 1.0:

65 Moby Thesaurus words for "thinning":
   Sanforizing, abatement, adulteration, atrophy, attenuation,
   attrition, blunting, consumption, contour plowing, cultivating,
   cultivation, culture, cutting, damping, deadening, debilitation,
   devitalization, diluent, dilution, dissolvent, dressing, drying,
   drying up, dulling, effemination, emaceration, emaciation,
   enervation, enfeeblement, etherealization, evisceration,
   exhaustion, extenuation, fallowing, fatigue, furrowing, harrowing,
   hoeing, inanition, languishment, listing, mitigation, parching,
   plowing, preshrinkage, pruning, rarefaction, reduction, relaxation,
   resolutive, resolvent, searing, shrinkage, shrinking, shriveling,
   slackening, softening, solvent, subtilization, tilling, wasting,
   weakening, weeding, withering, working