Aggressively self-assertive or arrogant in a way that is unpleasant or annoying.
Possibly from bump plus -itious (from Latin -itiosus), or perhaps influenced by fractious. Emerged in 18th-century English to describe obnoxiously confident behavior.
The '-tious' ending makes words sound pompous themselves—bumptious, ambitious, capricious—which is perfect because the word describes people who ACT pompous and self-important!
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