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1530s, from Latin adumbrationem (nominative adumbratio) "a sketch in shadow, sketch, outline," noun of action from past participle stem of adumbrare "to cast a shadow, overshadow, represent (a thing) in outline," from ad- "to" (see ad-) + umbrare "to cast in shadow," from PIE *andho- "blind, dark" (see umbrage).
"to outline, to sketch," 1580s, from Latin adumbratus "sketched, shadowed in outline," past participle of adumbrare "to represent (a thing) in outline" (see adumbration). Meaning "to overshadow" is 1660s. Related: Adumbrated; adumbrating.