Lewis & Short

sontĭcus, a, um, adj. [sons].

  1. I. Lit., dangerous, serious, critical, in the connection morbus sonticus, of a serious disorder that excuses one from duty, etc., Fragm. XII. Tab. ap. Fest. p. 290 Müll.; cf. Gell. 20, 1, 27; Dig. 21, 1, 65; 42, 1, 60; 2, 11, 2; 5, 1, 46; Plin. 36, 19, 34, § 142; Cincius ap. Gell. 16, 4, 4.
  2. II. Transf.: sontica causa, arising from a morbus sonticus; hence, in gen., serious, weighty, important, Cato ap. Fest. p. 344 Müll.; Naev. ib. p. 290; Tib. 1, 8, 51.