WordWeb Online

1. Of momentous or ominous significance
   "such a portentous...monster raised all my curiosity"
   - prodigious
    
2. Ominously prophetic
   - fateful, foreboding
    
3. Puffed up with vanity
   "a portentous and boastful manner"
   - grandiloquent, overblown, pompous, pontifical

See also: important, portent, portentously, pretentious, prophetic, prophetical, significant

Encyclopedia: Portentous