assertion (n.) Look up assertion at
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" (see series). By "joining oneself" to a particular view, one "claimed" or "maintained" it. From mid-15c. as "an unsupported statement."