IPA: /əˈɡrɛsɪvnəs/
KK: /əˈɡrɛsɪvnəs/
The quality or state of being forceful, hostile, or ready to attack. It often refers to a person's behavior that is assertive or combative.
His aggressiveness in negotiations often leads to better deals.
Aggressiveness is formed from "aggress-" (from Latin "aggressus", meaning to attack or approach) and "-iveness" (meaning the quality of). The word describes the quality of being inclined to attack or approach in a hostile manner.
Think of 'aggress-' meaning to 'attack' and '-iveness' indicating a quality — so aggressiveness is the quality of being inclined to attack.