verb (used with object) 1.to state with assurance, confidence, or force; state strongly or positively; affirm; aver: 

He asserted his innocence of the crime.

2.

to maintain or defend (claims, rights, etc.). 

3.to state as having existence; affirm; postulate: 

to assert a first cause as necessary.

Idioms 4.assert oneself, to insist on one's rights, declare one's views forcefully, etc.: 

The candidate finally asserted himself about property taxes.

[Originated from Latin]

1595-1605; < Latin assertus joined to, defended, claimed (past participle of asserere), equivalent to as- as- + ser- (see series) + -tus past participle suffix

 

 

Source: http://www.dictionary.com/browse/assert