asseverate (v.)

"affirm positively or solemnly," 1791, from Latin asseveratus/adseveratus, past participle of asseverare/adseverare "to affirm, insist on, maintain," from ad "to" (see ad-) + severus "serious, grave, strict, austere," which is probably from PIE root *segh- "to have, hold," on the notion of "steadfastness, toughness." Related: Asseverated; asseverating.