The Collaborative International Dictionary of English v.0.48:

Tremulous \Trem"u*lous\, a. [L. tremulus, fr. tremere to
   tremble. See Tremble.]
   1. Shaking; shivering; quivering; as, a tremulous limb; a
      tremulous motion of the hand or the lips; the tremulous
      leaf of the poplar.
      [1913 Webster]

   2. Affected with fear or timidity; trembling.
      [1913 Webster]

            The tender, tremulous Christian.      --Dr. H. More.
      [1913 Webster] -- Trem"u*lous*ly, adv. --
      Trem"u*lous*ness, n.
      [1913 Webster]
      [1913 Webster]

WordNet (r) 3.0 (2006):

tremulous
    adj 1: (of the voice) quivering as from weakness or fear; "the
           old lady's quavering voice"; "spoke timidly in a
           tremulous voice" [syn: quavering, tremulous]