Aunque la mona se vista de seda, mona queda. “Even if the monkey dresses in silk, she is still a monkey.”
A lesson on Mexican sayings in the Nueva Escuela Mexicana materials of the Ministry of Public Education explains it: “Significa que cada uno es como es, y no podemos ocultar o cambiar con mejoras meramente externas” (It means that each one is as one is, and we cannot hide or change it with merely external improvements).