If you add the suffix »∙heit« or »∙keit« to an adjective, you create a noun from that adjective (which btw. always is female):
dunkel → die Dunkelheit
fröhlich → die Fröhlichkeit
This is the same what you do with »∙ness« in English:
dark → the darkness
happy → the happiness
The noun you create this way is the name of the uncountable and universal concept of being dark/happy or whatever.
Der Mann im grauen Umhang verschwand lautlos in der Dunkelheit.
The man in the gray cloak disappeared silently into the darkness.
Sandra verbreitete Fröhlichkeit, wo immer sie auftauchte.
Sandra spread happiness wherever she appeared.
And in German, the noun Dummheit exactly fits into this scheme. So, a better translation of Dummheit might probably be stupidness:
dumm → die Dummheit
stupid → the stupidness
So, the sentence you quoted might be translated as:
Even gods fight in vain with stupidness.
Google told me, that this word exists, but it is rare. You more often use stupidity in English:
Even gods fight in vain with stupidity.
And the meaning of this sentence is:
The universal concept of being stupid, that manifests in many people, is so strong and indomitable, that even gods have no power over it. (In the sense of: Stupidity is such an indomitable power, that nobody has a chance against it.)
You can interpret this sentence in two ways (in German as well as in English):
- Gods can be stupid.
(Everybody can be stupid.)
- Gods are at the mercy of other people's stupidity.
(Everybody is at the mercy of other people's stupidity.)
Its the context that makes clear which one makes sense. I think, that it is #2 in Die Jungfrau von Orleans.
I read in your comment, that you also have troubles to understand the verb kämpfen (to fight). It is the same fight, that you use in
Walter is fighting fatigue.
Walter kämpft gegen die Müdigkeit.
If you watch Walter, you won't see any fight. He is just close to fall asleep, although he wants to say awake.