The Collaborative International Dictionary of English v.0.48:

Attenuate \At*ten"u*ate\, v. i.
   To become thin, slender, or fine; to grow less; to lessen.
   [1913 Webster]

         The attention attenuates as its sphere contracts.
                                                  --Coleridge.
   [1913 Webster] Attenuate

The Collaborative International Dictionary of English v.0.48:

Attenuate \At*ten"u*ate\, Attenuated \At*ten"u*a`ted\, a. [L.
   attenuatus, p. p.]
   1. Made thin or slender.
      [1913 Webster]

   2. Made thin or less viscid; rarefied. --Bacon.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Attenuate \At*ten"u*ate\, v. t. [imp. & p. p. Attenuated; p.
   pr. & vb. n. Attenuating.] [L. attenuatus, p. p. of
   attenuare; ad + tenuare to make thin, tenuis thin. See
   Thin.]
   1. To make thin or slender, as by mechanical or chemical
      action upon inanimate objects, or by the effects of
      starvation, disease, etc., upon living bodies.
      [1913 Webster]

   2. To make thin or less consistent; to render less viscid or
      dense; to rarefy. Specifically: To subtilize, as the
      humors of the body, or to break them into finer parts.
      [1913 Webster]

   3. To lessen the amount, force, or value of; to make less
      complex; to weaken.
      [1913 Webster]

            To undersell our rivals . . . has led the
            manufacturer to . . . attenuate his processes, in
            the allotment of tasks, to an extreme point. --I.
                                                  Taylor.
      [1913 Webster]

            We may reject and reject till we attenuate history
            into sapless meagerness.              --Sir F.
                                                  Palgrave.
      [1913 Webster]

WordNet (r) 3.0 (2006):

attenuate
    adj 1: reduced in strength; "the faded tones of an old
           recording" [syn: attenuate, attenuated, faded,
           weakened]
    v 1: weaken the consistency of (a chemical substance) [syn:
         rarefy, attenuate]
    2: become weaker, in strength, value, or magnitude

Moby Thesaurus II by Grady Ward, 1.0:

95 Moby Thesaurus words for "attenuate":
   Sanforize, abate, adulterate, alleviate, attenuated, baptize, bate,
   blow off, blunt, cast forth, clear away, constrict, consume,
   contract, cramp, cripple, cut, damp, dampen, deaden, debilitate,
   deflate, dematerialize, devitalize, dilute, disable, disembody,
   dispel, dissipate, dissolve, drive away, dry up, dull, ease,
   emacerate, emaciate, enervate, enfeeble, etherealize, evaporate,
   eviscerate, exhaust, expand, extenuate, gruel, irrigate, lay low,
   lessen, macerate, mitigate, parch, preshrink, rare, rarefied,
   rarefy, rattle, reduce, reedy, remit, sap, sear, shake, shake up,
   shrink, shrivel, slacken, slender, slight, slim, soften up,
   spiritualize, subtile, subtilize, subtle, tenuous, thin, thin away,
   thin down, thin out, twiggy, unbrace, undermine, unman, unnerve,
   unstrengthen, unstring, volatilize, waste, waste away, water,
   water down, weaken, weazen, wither, wizen