The quality or state of being darksome; a condition of darkness, gloom, or dreariness.
From 'darksome' + the '-ness' suffix, creating an abstract noun. This is a double suffix construction (dark + -some + -ness) that expresses the quality of being darksome.
This is exactly the kind of word Shakespeare's contemporaries would have loved—double and triple suffix construction was seen as sophisticated and poetic, even if it sounds over-the-top today.
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