Webster's English Dictionary

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insidious

in.sid.i.ous \in-'sid-e--*s\ aj [L insidiosus, fr. insidiae ambush, fr. 
   inside-re to sit i]n, sit on, fr. in- + sede-re to sit - more at SIT 1a: 
   awaiting a chance to entrap : TREACHEROUS  1b: harmful but enticing : 
   SEDUCTIVE  2a: having a gradual and cumulative effect : SUBTLE  of a 
   disease  2b: developing so gradually as to be well established before 
   becoming appa rent - in.sid.i.ous.ly av