yeap, burro/donkey is like the dumbest someone can be........... back in the days when my dad attended grammar school, a pusnishment (for those kids who had academic issues) would be to stand in a corner of the classroom and wear a black cone shaped hat that read the word "Burro." Can you imagine being the official "Burro" of the class???
But " Que burro eres!" "No seas burro!" "Pero que burro/a" is slang too....people use it all the time and it doesnt' mean the person you're talking to is a real "burro"...LOL...I use it all the time
"estar crudo" is hangover