Lewis & Short

ab-sŏnus, a, um, adj.

  1. I. Deviating from the right tone, discordant, dissonant, inharmonious: sunt quidam ita voce absoni, ut … in oratorum numerum venire non possint, Cic. de Or. 1, 25, 115: vox absona atque absurda, id. ib. 3, 11, 41.
    Hence,
  2. II. In gen., not harmonizing with a thing, not accordant with, unsuitable, incongruous; constr. with ab or ( = alienus) with dat. or absol.: nec absoni a voce motus erant, Liv. 7, 2: nihil absonum fidei divinae originis fuit, id. 1, 15: fortunis absona dicta, Hor. A. P. 112.
    Absol.: nihil absonum, nihil agreste, Quint. 6, 3, 107; cf. id. 12, 10, 32.
    Adv.: absŏnē, discordantly, incongruously, Gell. 15, 25; App. Mag. p. 277.