|1.||stealthy, subtle, cunning, or treacherous|
|2.||working in a subtle or apparently innocuous way, but nevertheless deadly: an insidious illness|
|[C16: from Latin insidiōsus cunning, from insidiae an ambush, from insidēre to sit in; see |
insidious in·sid·i·ous (ĭn-sĭd'ē-əs)
Being a disease that progresses with few or no symptoms to indicate its gravity.