Lewis & Short

1. rūdus, ĕris, n., stones broken small and mingled with lime for plastering walls, paving floors, etc.

  1. I. In gen., Vitr. 7, 1: rudus inicere solo, Plin. 36, 25, 62, § 186; Pall. 1, 9, 4; 11, 2 Mai: aedificia tecta rudere aut pavimentis, Auct. B. Alex. 1, 3: rudere, non tegulis teguntur, Auct. B. Hisp. 8, 2: pingue, i. e. rich soil, Col. poët. 10, 81.
  2. II. In partic., old rubbish, of the stones, plaster, etc., of decayed buildings (rare and not ante-Aug.): ruderi accipiendo Ostienses paludes destinabat, Tac. A. 15, 43.
    In plur.: alveum Tiberis laxavit ac repurgavit, completum olim ruderibus, Suet. Aug. 30; id. Vesp. 8.