"offensively assertive," 1803, probably a jocular slang coinage from bump on the pattern of fractious, etc. Related: Bumptiously; bumptiousness.
Entries linking to bumptious
bump n.
1590s, "protuberance caused by a blow;" 1610s as "a dull-sounding, solid blow;" see bump (v.). The dancer's bump and grind is attested from 1940. To be like a bump on a log "silent, stupidly inarticulate" is by 1863, American English.