In a fetid manner; with a foul or putrid smell; stinkingly.
From Latin 'foetidus' (stinking) plus the English adverbial suffix '-ly' (in the manner of). This adverb form allows the descriptor to modify verbs and other adjectives.
The adverb 'fetidly' is rarely used in modern English—we tend to say 'smells fetid' rather than 'smells fetidly.' It's a reminder that language evolves and some words become obsolete versions of themselves!
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