Fex urbis lex orbis is a Latin saying, meaning "Dregs [classical Latin faex] of the city, law of the world", that is, the lowest class of citizens determines how the world works.
First written by St. Jerome, the phrase is often erroneously attributed to Victor Hugo, who quotes it ironically at the beginning of Volume V of Les Misérables while advising one to be careful in labeling social groups:
the beggars were the making of Holland; the populace saved Rome more than once; and the rabble followed Jesus Christ.
There is no thinker who has not at times contemplated the magnificences of the lower classes.
It was of this rabble that St. Jerome was thinking, no doubt, and of all these poor people and of all these vagabonds and of all these miserable people whence sprang the apostles and the martyrs, when he uttered this mysterious saying: "Fex urbis, lex orbis," — the dregs of the city, the law of the earth.