Literary and euphemistic expression meaning 'to die', literally 'the worlds of the living and the dead are separated'. Used in classical or poetic contexts.
Polite and common euphemism for 'to die'; 幽明相隔てる is far more formal and rare, used mainly in classical or poetic writing.
Etymology
From 幽明 (the worlds of the dead and the living) + 相 (mutually) + 隔てる (to separate). The exact historical derivation is uncertain, but the phrase is a classical literary expression.
