Someone bumptious is cocky, aggressive, and loud. Bumptious people jump ahead of everyone in line at the ice cream truck and steal subway seats from pregnant women.
It's good to speak your mind and stand up for yourself, but it's not good to be bumptious. When you're bumptious, you're obnoxious. Someone who dominates a conversation without listening to others is bumptious. Someone who takes more than their fair share of something is bumptious. To be bumptious is to be selfish and annoying. Bumptious people are often arrogant and usually thoughtless, and bumptious behavior shows no concern for other people.