Extremely or excessively foul; very filthy, disgusting, or morally corrupt (archaic or rare usage).
From 'foul' + the suffix '-some,' which means 'full of' or 'characterized by excessive amounts of.' This formation parallels words like 'loathsome' and was more productive in Middle English.
The suffix '-some' (meaning 'full of' or 'tending toward') is less common in modern English than in Middle English, but we still use it for words like 'handsome,' 'gruesome,' and 'awesome'—it's an ancient way of intensifying descriptors.
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