Davy Jones's locker; Davy Jones's; Davy's locker
noun
1
the last resting place of those lost at sea; the sea. Davy Jones has been used as a personification for the 'spirit of the sea' since 1751, his locker is mentioned in The Journal of Richard Cresswell, 1774–7; the etymology, however, is another mystery of the deep. Jones may arise from Jonah (and his biblical adventures at sea), Davy may have been added by Welsh sailors in honour of St David UK, 1777.
2
a door knocker. Rhyming slang UK, 1992