asseveration (n.)

"an emphatic assertion," 1550s, from Latin asseverationem (nominative asseveratio) "vehement assertion, protestation," noun of action from past-participle stem of asseverare/adseverare "affirm, insist on," 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."