Its origin is the verb pragnąć (to desire, to long for) or the noun pragnienie (desire, but it also means thirst).
Cognates include Ukrainian прагнути/prahnuty (to desire, to long for) and прагнення/prahnennia (desire, wish) and Czech prahnout (to yearn), Belarusian прага/praha (desire, thirst). Their common origin is the reconstructed Proto-Slavic verb *pragnǫti, which possibly originates from the reconstructed Proto-Indo-European *preg- (to yearn, to covet).
„Ona jest spragniona” is perfectly fine sentence. It might mean that she is in some degree dehydrated and needs to drink something to satisfy the basic biological needs. It could also be used various figurative contexts, like „spragniona rozrywki.”
„Chce coś do picia” means "wants something to drink". It can be more indirect way to express the above, but it doesn't automatically imply the literal thirst. She might simply want to drink something because she likes the taste.