assertion (n.) Look up assertion at Dictionary.com
early 15c., assercioun, "a declaration," from Middle French assertion (14c.) or directly from Late Latin assertionem (nominative assertio), noun of action from past participle stem of Latin asserere "claim rights over something, state, maintain, affirm," from ad "to" (see ad-) + serere "join" (see series). By "joining oneself" to a particular view, one "claimed" or "maintained" it.