sheet anchor
Concise Oxford English Dictionary © 2008 Oxford University Press:
sheet anchor
▶noun
- 1 an additional anchor for use in emergencies.
- 2 a dependable person or thing relied upon in the last resort.
– origin C15: perh. rel. to obs. shot, denoting two cables spliced together, later influenced by sheet2.
'sheet anchor' also found in these Oxford entries:

