Made dark or darker; reduced the amount of light in something, or metaphorically made sad or troubled.
From 'dark' (Old English 'deorc,' lacking light) plus the verb suffix '-en' (to cause to be), plus past tense '-ed.' The word has applied to both light and mood since Old English.
The metaphor of 'darkened mood' or 'darkened thoughts' appears in nearly every language because our brains are wired to associate darkness with danger and difficulty—it's not just poetry, it's how human perception actually works.
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