as·sert / əˈsərt/ • v. [tr.] state a fact or belief confidently and forcefully: the company asserts that the cuts will not affect development he asserted his innocence. ∎  cause others to recognize (one's authority or a right) by confident and forceful behavior: the good librarian is able to assert authority when required. ∎  (assert oneself) behave or speak in a confident and forceful manner: it was time to assert himself.DERIVATIVES: as·sert·er n.

assert XVII. f. assert-, pp. stem of L. asserere, f. AS- + serere join.
So assertion XV. — F. or L. assertive XVII (the corr. adv. is XV). — F. assertif.