sheet anchor

noun3

1. •
   A large, spare anchor used in an emergency.

   • The Ship having thus depth of water again, and come into a place of some convenient anchourage, our first anchour not holding us, we let fall our sheet anchour […] — 1661
2. •
   A source of help in times of danger; last resort.

   • “[…] If you are afeard of goblins, brother, put your trust in the Lord, and he’ll prove a sheet-anchor to you.” — 1762
3. •
   A batsman who provides dependable defence while a series of other batsmen score rapidly.