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

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