Source : Webster's Revised Unabridged Dictionary (1913)

Sheet anchor \Sheet" an"chor\ [OE. scheten to shoot, AS.
   sce['o]tan; cf. OE. shoot anchor. See {Shoot}, v. t.]
   1. (Naut.) A large anchor stowed on shores outside the waist
      of a vessel; -- called also {waist anchor}. See the Note
      under {Anchor}.

   2. Anything regarded as a sure support or dependence in
      danger; the best hope or refuge.

Source : WordNet®

sheet anchor
     n : spare anchor for use in emergency [syn: {waist anchor}]