“Skin for skin”
'Skin for skin!' Satan replied. 'A man will give up all he owns in exchange for his life.' (Job 2:4 BSB)
What it sounds like today
A cryptic phrase about trading skin.
The cultural key
To a modern ear 'skin for skin' is nonsense. It is an old trader's idiom for an even swap, hide traded for hide, one thing given to get its equal. Satan weaponizes it: he claims Job's uprightness is nothing but calculated self-interest, that a man will hand over everything he has, and everyone he has, as long as his own 'skin,' his life, stays safe. Job's first round of losses only cost him property and children, all of it, in Satan's cynical accounting, external 'skin' Job would trade away to preserve himself. The real test, Satan argues, is to 'stretch out Your hand and strike his flesh and bones.' The verse glosses its own idiom in the next line, 'a man will give up all he owns in exchange for his life,' so the sense is anchored: hit Job's own body and the true motive will show. The phrase is a cynic's theory of human loyalty, not a riddle.
Recovered meaning
'Skin for skin' is a bartering idiom for an even trade. Satan uses it to argue that Job's devotion is mere self-interest, that a man will surrender anything to save his own life, so the true test is to strike Job's own flesh.