1424, from O.Fr.
assertion (14c.), from L.L.
assertionem (nom.
assertio), noun of action from L.
asserere "claim rights over something, state, maintain, affirm," from
ad- "to" +
serere "join" (see
series). By "joining oneself" to a particular view, one "claimed" or "maintained" it.
Assertiveness "tendency toward self-assertion" is from 1881.