Source : Webster's Revised Unabridged Dictionary (1913)

Quaver \Qua"ver\, v. i. [imp. & p. p. {Quavered}; p. pr. & vb.
   n. {Quavering}.] [OE. quaven to shake, to tremble; cf. LG.
   quabbeln to shake, to be soft, of fat substances, quabbe a
   fat lump of flesh, a dewlap, D. kwabbe, and E. quiver, v.]
   1. To tremble; to vibrate; to shake. --Sir I. Newton.

   2. Especially, to shake the voice; to utter or form sound
      with rapid or tremulous vibrations, as in singing; also,
      to trill on a musical instrument

Source : WordNet®

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