Superlative form of dark; having the least light or the most intense darkness. Can refer to the most grim or hopeless situation.
From Old English deorc 'dark, obscure' + -est superlative suffix. The metaphorical sense of 'most gloomy or evil' parallels the physical meaning throughout English history.
The phrase 'darkest hour' became famous from Churchill's speech, but astronomically, the darkest time isn't midnight - it's about 2 AM when even the faintest atmospheric glow has faded.
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