As adjectives the difference between portentous and unpropitious
is that portentous is of momentous or ominous significance while unpropitious is not propitious; unfavourable, untimely.
portentous
English
Adjective
(
en adjective)
Of momentous or ominous significance.
Ominously prophetic.
Puffed up with vanity.
Related terms
* portent
unpropitious
English
Adjective
(
en adjective)
not propitious; unfavourable, untimely
Derived terms
* unpropitiously
* unpropitiousness