Fex urbis lex orbis is a Latin saying, meaning "Dregs [classical Latin faex] of the city, law of the world". In other words, the desires and needs of the lowest class of citizens actually determine how the world works, by the sheer force of their numbers.
First said by St. Jerome, it is often erroneously attributed to Victor Hugo, who quotes it with approval in Les Misérables.
The similar words urbis and orbis also appear together in the phrase Urbi et Orbi.