One who supports, affirms, defends, or vindicates; a champion(noun)
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Use "assertor" in a sentence
"With a swifter pace than his half-pay leisure usually encouraged, or than his habitual dignity permitted, Captain MacTurk cleared the ground betwixt the Spring and its gay vicinity, and the ruins of the Aultoun, where reigned our friend Meg Dods, the sole assertor of its ancient dignities."
"It is, preventively, the assertor of its own rights; or remedially, their avenger."
"He appears to have become a convert to the doctrine of the Quakers, or Friends, and a great assertor of their peculiar tenets."