early 15c., assercioun, "a declaration, confirmation" from Old French assercion (14c.) or directly from Late Latin assertionem (nominative assertio), noun of action from past-participle stem of Latin asserere/adserere "to claim, lay claim to, appropriate," from ad "to" (see ad-) + serere "join together, put in a row" (from PIE root *ser- (2) "to line up"). By "joining oneself" to a particular view, one "claimed" or "maintained" it. From mid-15c. as "an unsupported statement."
