Dark, shadowy, or gloomy; existing or occurring in darkness or dim light.
From 'dark' with the suffix '-ling' (which can mean 'somewhat' or 'in the manner of'), creating an archaic and poetic adjective. It appears extensively in Renaissance literature and Milton's works.
The beautiful phrase 'darkling thrush' from Thomas Hardy's poem is probably the most famous modern use—and 'darkling' gives the image a mysterious, ancient quality that plain 'dark' doesn't.
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