The Adonis muscle does not exit, but it does refer to the muscles of the shoulder, and to some extent the upper chest and upper arms. These Adonis muscles, shoulder muscles, help give men the Adons shape or Adonis index which is the ratio of ones measurements around the shoulders compared to ones waist. This ideal shape or Adonis index is 1.618.
Example: if you are in reasonably good condition (say 8 -10% fat) and have a waist of 30 inches:
You should have 30 X 1.618 = 48.54 inches around your shoulders.
This is the shape on averarge that women find most attractive in men. It was originaly found in the sculptures of ancient Greece, but has been confirmed by those doing clothing research. (Ever wander why mens sports coats have padding in the shoulders? ) Similar measurements can be found at ideal body measurements for men. Also this ratio seems to be compair faforably with what women say they like in the blogs. The larger ratios as seen on body builders does not seem to be as attractive to women.
By the way it is not necessary to get the exact 1.618 Adonis index, as a matter of fact people who worry too much about having this ideal male body are said to suffer from the "Adonis Complex." Just lose weight in your waist and work on your adonis muscle and you will be ok.
Adonis was one of may Greek gods. In all the stories Adonis was young and had many lovers so was pictured with an ideal body in all the Greek art work.
Here is a picture of Adonis.