Lewis & Short

ăcerbo, āvi, ātum, 1, v. a. [acerbus] (vox Vergil.).

  1. I. To make harsh or bitter, to embitter; lit. and trop. (very rare): gaudia, Stat. Th. 12, 75: mortem, Val. Fl. 6, 655.
    Hence in an extended sense,
  2. II. To augment or aggravate any thing disagreeable (cf. acuo): formidine crimen acerbat, Verg. A. 11, 407: nefas Eteoclis, Stat. Th. 3, 214.