A shackle key or metal marlinspike are useful tools for loosening a tight nut.
From the en.wikipedia.org
Its common name is thought to derive from its resemblance to a sailor's marlinspike.
From the en.wikipedia.org
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Marlinespike: a pointed iron hand tool that is used to separate strands of a rope or cable (as in splicing)
Marlinspike Hall (French: Le chu00E2teau de Moulinsart or French: Moulinsart) is Captain Haddock's country house and family estate in The Adventures of Tintin, the comics series by Belgian cartoonist Hergu00E9.
Also marline spike, marlin spike, marlingspike, see marlinespike
A tool for opening the strands of a rope while splicing.