Hacer de chivo los tamales

This is an old yet sound expression. Hacer de chivo los tamales (ah-cer the-CHEE-boh los tah-MA-less) literally means "to make the tamales with goat". It means to cheat or not keep one's word. Tamales are supposed to have pork or chicken, not goat.

In some contexts is similar to transar and in others to pasarse de lanza. It is not an offensive expression, but since it involves one of Mexican's favorite dishes, it is a really colorful one.

Example:

Doña Macaria: Dicen que desde que su marido se fue al otro lado anda muy contentota.
Doña Rutila: A mí se me hace que le hizo de chivo los tamales al pobre.

Macaria: They say after he left for the States she's been quite happy.
Rutila: I think she's cheating on that poor man.

In Mexico City you have to try the tamales out of the steaming pots of the street puestos. Nothing makes a better morning snack that a tamal.