The Collaborative International Dictionary of English v.0.48:

Asseverate \As*sev"er*ate\, v. t. [imp. & p. p. Asseverated;
   p. pr. & vb. n. Asseverating.] [L. asseveratus, p. p. of
   asseverare to assert seriously or earnestly; ad + severus.
   See Severe.]
   To affirm or aver positively, or with solemnity.
   [1913 Webster]

   Syn: To affirm; aver; protest; declare. See Affirm.
        [1913 Webster]

WordNet (r) 3.0 (2006):

asseverate
    v 1: state categorically [syn: assert, asseverate,
         maintain]