Green’s Dictionary of Slang

safe (and sound) n.

the ground.

1992		R. Puxley Cockney Rabbit.
2001		M. Coles Bible in Cockney 20: Noah [...] ’ad a good butcher’s around, and ’e could see that the safe was getting dry.
2002		M. Coles More Bible in Cockney 10: A little ol’ plant taking root in the dry safe-and-sound.