Lewis & Short

assĕvērātĭo (ads-), ōnis, f. [assevero].

  1. I. An earnest pursuit of any thing; hence,
    1. A. Of discourse, a vehement assertion, affirmation, asseveration: omni tibi adseveratione affirmo, Cic. Att. 13, 23: confirmatio est nostrorum argumentorum expositio cum adseveratione, Auct. ad Her. 1, 3; so Quint. 4, 2, 94; 11, 3, 2; Plin. Pan. 67; Tac. A. 6, 2; 4, 42; 4, 52.
    2. B. In Tac., of actions, a persevering earnestness, vehemence, rigor: igitur multā adseveratione … coguntur patres, etc., Tac. A. 4, 19: accusatio tamen apud patres adseveratione eādem peracta, id. ib. 2, 31.
  2. * II. In the old gram. lang., a strengthening part of speech, a word of emphasis: adiciebant et adseverationem, ut heu, Quint. 1, 4, 20: (adseverat heu, dum miserabili orationi ipsius, qui dicit dolorem, adjungit, Spald.).