Treacherous; stealthy; denoting a disease that progresses gradually with inapparent symptoms. [L. insidiosus, cunning, fr. insidiae (pl.), an ambush]
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in·sid·i·ous in-'sid-ē-əs adj developing so gradually as to be well established before becoming apparent <an \insidious disease>
in·sid·i·ous·ly adv
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in·sid·i·ous (in-sidґe-əs) [L. insidiosus deceitful, treacherous] coming on in a stealthy manner; of gradual and subtle development.Medical dictionary. 2011.