If you do not find the Latin phrase you are looking for on this page, here are more Latin phrases about stupidity and ignorance:
Experientia docet (Tacitus)
damnant quod non intellegunt
They pass judgement on what they do not comprehend.
(Cicero - Roman statesman, lawyer, philospher and orator)
ne supra crepidam sutor iudicaret (Pliny the Elder)
Shoemaker should not judge above the sandal.
I know not what...
nolumus leges Angliae mutari (King Charles I of England)
We do not want the laws of England to change.