Lewis & Short

lăcūnar, āris (nom. lacūnārĭum, Isid. Orig. 15, 8, 6;

  1. I. gen. plur. lacunariorum for lacunarium, Vitr. 4, 3, 1 al.; dat. plur. lacunariis, id. 5, 2), n. [lacuna], a wainscoted and gilded ceiling of an unvaulted chamber, a panel-ceiling, a ceiling (so called from its sunken spaces; class.), Vitr. 7, 2: non ebur neque aureum Mea renidet in domo lacunar, Hor. C. 2, 18, 2: gladium e lacunari seta equina aptum demitti jussit, Cic. Tusc. 5, 21, 62: primus lacunaria pingere instituit (Polygnotus), Plin. 35, 11, 40, § 124.
    Prov.: spectare lacunar, to gaze at the ceiling, to be wilfully blind, Juv. 1, 56.
  2. II. Plur.: lăcūnārĭa, ōrum (-arium, App. Flor. 18, p. 83), n., panels of the under surface of a cornice, Vitr. 4, 3, 1; 7, 2, 2; 5, 2, 1; Plin. 35, 11, 40, § 124.