This expression means "thirteen of something". It comes from a practice of bakers in medieval times. They used to include an extra loaf of bread in a dozen, so as to avoid being fined for selling less weight than it was permitted. A dozen had to have a certain weight regulated by law. To be sure that they weren't breaking the law, bakers included one (or two) more pieces of bread. The phrase began to be used in the 17th century to refer to a group of  thirteen items.

Esta expresión significa "trece de algo". Proviene de la práctica de los panaderos en la época medieval. Solían incluir un pancito extra con la docena, para evitar ser multados por vender menos peso del permitido. Una docena debía tener un peso determinado regulado por la ley. Para asegurarse de no estar violando la ley, los panaderos incluían uno (o dos) pancitos más. La frase comenzó a usarse en el siglo XVII para referirse a un grupo de trece elementos.