Lewis & Short

insĭdĭātor, ōris, m. [insidior],

  1. I. a soldier lying in ambush, Hirt. B. G. 8, 18.
  2. II. Transf., one who lies in wait, a lurker, waylayer (rare but class.): viae, Cic. Cat. 2, 12, 27: in foro collocatur, id. Mil. 7, 19: imperii, Nep. Reg. 2.