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