In a drear or gloomy manner; with an air of dismalness or depression.
From drear + -ful (Old English ful, meaning 'full of') + -ly (adverbial suffix). This creates an adverb from an adjective that already contains the -ful suffix, showing how English allows multiple levels of affixation.
Stacking suffixes like this (drear-ful-ly) is perfectly legal in English, but it sounds archaic now—modern writers would more likely say 'drearily' or 'gloomily,' showing how word formation changes with speaker preferences.
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