sheet anchor

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Nautical A large extra anchor intended for use in an emergency.
• noun A source of aid in time of emergency or danger.

from The Century Dictionary.

• noun One of two anchors, carried on shores in the waist, outside, abaft the fore-rigging, and used only in cases of emergency.
• noun Hence Figuratively, chief dependence; main reliance; last resort.

from the GNU version of the Collaborative International Dictionary of English.

• (Naut.) A large anchor stowed on shores outside the waist of a vessel; -- called also waist anchor. See the Note under anchor.
• Anything regarded as a sure support or dependence in danger; the best hope or refuge.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun nautical a large, spare anchor used in an emergency
• noun by extension a source of help in times of danger
• noun cricket a batsman who provides dependable defence while a series of other batsmen score rapidly

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun spare anchor for use in emergency