Technically a tumbaburros is a large bumper that would be used to knock donkeys out of the way when driving. Sometimes called a cattle guard bumper for protecting your grill if you happen to hit a large animal.
But that is not what is being referred to when a Mexican talks about a tumbaburros. They are referring to large reference books and particularly the Real Academia Española dictionary. The idea that it is big enough to be used as one of these types of bumpers. Or just hitting a donkey with this dictionary would be enough to knock him over.
I already knew this word, but what got me to thinking about it was when walking through a mall yesterday I saw a restaurant called El Tumbaburros. Their tag line was “La Real Academia del Taco.”