The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. Anything regarded as a sure support or dependence in
      danger; the best hope or refuge.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sheet anchor
    n 1: spare anchor for use in emergency [syn: sheet anchor,
         waist anchor]