身から出た錆
Of rusty swords and personal fault.
... 「身」は刀の鞘に納まっている部分を指す「刀身」のことである。
刀の手入れを怠ると刀身から錆が出て、いざという時に使い物にならず、自分自身の命（身）を落とすことになる。
そこから、刀の身と自分自身の身をかけ、自分の犯した罪のために自分自身が苦しむことを「身から出た錆」と言うようになった。また、刀身から出た錆は表面だけではなく、刀自体を腐らせてしまうので、その意味でも、取り返しがつかないことをしたという悔やみが込められた言葉である。
'Mi' (身) refers to the part of the sword sheathed within the scabbard (the sword blade).
When a man neglects his katana and fails in its upkeep, the sword will rust, which renders it useless. In dire times where the sword is most required, the man will thus lose his life (because of his own negligence and unpreparedness).
The proverb likens the sword's body to one's own, and this is how the proverb came to mean 'one's despairing over something that was his own fault to begin with'.
The proverb can also be extended to include nuances of condolence for (unfortunate) situations where there is no hope for recovery, from the idea that rust envelops the sword blade whole (and not just its surface).