Basically it comes from this:

http://en.wikipedia.org/wiki/Ernst_Kretschmer

Kretschmer is also known for developing a classification system that can be seen as one of the earliest exponents of a constitutional (the total plan or philosophy on which something is constructed) approach. His classification system was based on three main body types: asthenic/leptosomic (thin, small, weak), athletic (muscular, large–boned), and pyknic (stocky, fat). (The athletic category was later combined into the category asthenic/leptosomic.) Each of these body types was associated with certain personality traits and, in a more extreme form, psychopathologies. Kretschmer believed that pyknic persons were friendly, interpersonally dependent, and gregarious. In a more extreme version of these traits, this would mean for example that the obese are predisposed toward manic-depressive illness. Thin types were associated with introversion and timidity. This was seen as a milder form of the negative symptoms exhibited by withdrawn schizophrenics. However, the idea of the association of body types with personality traits is no longer influential in personality theory.

In 1926 he became the director of the psychiatric clinic at Marburg University.

What about endomorph, ectomorph and mesomorph?

Endomorph---stocky build
Ectomorph---tall long
Mesomorph---huskey muscular

Practically synonim to Kretschtmer's typology, but without psychological implications. (outdated anyway)
But Hydra, what do you think of this definition coming from Encarta:
The whole thing:
Mesomorph : a husky muscular body or somebody who has such a body.

The 'or' in this sentence makes me chuckle. I do not understand the 'or 'in this phrase. It's like someone is hiding
such a body in a closet or something.

What about this sentence ? Don't you think it is a weird definition?

It does make the husky body sound like a spare. As opposed to pyknik which just has a spare tire.

>a husky muscular body or somebody who has such a body.

Ha, ha!

A very unusual distinction is being made here, you're right, and with ridiculous implications: The second refers to someone having a husky, muscular body; the first to a husky, muscular body lacking a someone—an automatized hulk of muscle without a mind.
Reminds me of some of the people I see at the gym.

Warning: This thread is now in danger of entering into the Cartesian mind/body dichotomy.

a husky muscular body or somebody who has such a body
I--already knowing the meaning of mesomorph, mind you--saw this as going from the general to the specific, as in: mesomorphs have thick bodies OR "That guy really shouldn't wear tiny swim trunks--he's a mesomorph."
However, I can see where the def. might make you wonder what's in a certain person's closet...