A euphemism. Commonly used by arrogant, aggressive and disrespectful people to put a positive spin on their behaviour.
Also used by normal folk to describe primitive beings who feel it necessary to crush or dominate everyone and everything around them.
Linda: I'm not being a bitch
! I'm being assertive... I have every right to tell you what to do and how to live if I disagree with it...
Mike: Uhh... you're a biatch
Garry: That new manager's quite assertive...
Pat: Yep, quite assertive.... so assertive I want to go home and puke
(spoken with delightful sarcasm)