Lewis & Short

ŏpāco, āvi, ātum, 1, v. a. [onyx], to cover with shade, to shade (class.; syn.: obscuro, obumbro).

  1. I. Lit.; platanus ad opacandum hunc locum patulis est diffusa ramis, Cic. de Or. 1, 7, 28, cf. id. Fragm. ap. Macr. S. 6, 4: ubi pinguem dives opacat Ramus humum, Verg. A. 6, 195: humum taxus opacat, Luc. 6, 645: (sol terras) modo his modo illis ex partibus opacat, Cic. N. D. 2, 19, 49.
    1. B. Transf., to cover (poet.): opacat flore lanugo genas, Pac. ap. Fest. s. v. genas, p. 94 Müll.: opacat tempora pinus, Sil. 13, 331.
  2. * II. Trop., to darken, obscure, Aug. Mor. Eccl. Cath. 1, 2.