In Spain and some Spanish colonies: the guild of owners of transhumant livestock, especially merino sheep, founded in 1273 by Alfonso X to enforce the payment of certain taxes, and having great political power.
The society enjoyed royal protection until after the reign of Philip IV (1621–65), but declined after 1758, when the taxes it collected were abolished. It ceased to exist in 1836.
Early 19th century. From Spanish Mesta (14th or 15th century in this sense, earlier in senses ‘group of animals owned communally or by several owners’ and ‘a confluence of streams’) from post-classical Latin mixta (short for animalia mixta mixed animals), specifically use of neuter plural of classical Latin mixtus.