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):

thinned
    adj 1: mixed with water; "sold cut whiskey"; "a cup of thinned
           soup" [syn: cut, thinned, weakened]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "thinned":
   adulterated, airy, attenuate, attenuated, cut, dilute, diluted,
   ethereal, fine, flimsy, gaseous, insubstantial, rare, rarefied,
   reduced, slight, subtile, subtle, tenuous, thin, thinned-out,
   uncompact, uncompressed, unsubstantial, vaporous, watered,
   watered-down, windy