Moving downward into water or deeper into something, or the feeling of one's mood or hope going down.
From Old English 'sincan' (to sink, fall). The Germanic root is related to 'sunk' and ultimately describes the downward motion of something becoming submerged.
Your stomach 'sinking' when you're afraid and an actual ship sinking use the exact same metaphor—our bodies literally feel like they're dropping when we're scared, which is why this one word works for both!
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