Lewis & Short

insĭdĭōsus, a, um, adj. [insidiae], cunning, artful, deceitful, dangerous, insidious (class.).

  1. I. Of persons: quis insidiosior unquam fuit? Cic. Verr. 2, 2, 78, § 192.
  2. II. Of inanim. and abstr. things: Capraria insidiosa naufragiis, Plin. 3, 5, 11, § 78: pocula Circes, Ov. M. 14, 294: verba, id. H. 20, 212: clementia, Cic. Att. 8, 16, 2.
    Sup., Plin. 29, 1, 8, § 28.
    Adv.: insĭdĭōsē, cunningly, deceitfully, insidiously: in gratiam rediit cum illo, Cic. Rab. Post. 12, 33: me insidiosissime tractavit, id. Q. Fr. 1, 3, 8.