1st_ Reduced "master" shape. It is an archaism and today a vulgarism. See Master . 2nd_ In Greek mythology Mestra was the daughter of Erisiton, and had the ability to change her appearance at will.
In Greek mythology, Mestra was a daughter of Soniton, who was king of Thessaly. It is also the name of a Spanish company that supplies equipment for use in Dentistry.