Sully has this etymological development: Probably from Middle French souiller ('to soil, dirty'), from Old French soillier, ultimately related to Latin suculus ('little pig') or to Vulgar Latin *suliare; influenced by Old English sylian ('to defile'). Important recorded forms include souiller (Middle French) and soillier (Old French). Morphologically, the -ly ending turns the base adjective or participial form into an adverbial manner word, or preserves an older adjectival pattern where the part of speech allows it. The wording preserves that uncertainty: the older history is best read as a cautious account rather than a fully settled chain. The modern sense continues the older lexical line without requiring a separate origin. In current use it covers to damage or tarnish the purity or reputation of something. The wider family includes derived forms such as sullied and sullying and related words such as soil.