Having gone beneath the surface of water; past participle of sink, often used figuratively to describe being in a hopeless situation.
From Old English 'sincan' meaning to go down or descend. The past participle 'sunk' developed alongside 'sunken,' with 'sunk' becoming more common in predicative uses and figurative expressions.
The phrase 'sunk cost' brilliantly uses the finality of drowning to describe unrecoverable investments - just as something truly sunk cannot be easily retrieved, money already spent cannot be unspent. This metaphor helps economists explain why we shouldn't chase past losses with future resources.
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