A pyknic person is short and fat. Nothing to do with over-indulgence in al-fresco meals, though it is said the same way as picnic.
In the early years of the twentieth century the German psychiatrist Ernst Kretschmer examined criminals to try to tie their physical shape and constitution to their personalities and mental illnesses. Tall and thin ones he called asthenic (or leptosomic) and considered them to be the sort that commits fraud and petty theft. A second set were athletic types with well-developed muscles — unsurprisingly, he concluded these were more likely to be violent. The third sort were the pyknic ones, who seemed to be a mixture of the other two kinds so far as their criminal tendencies were concerned. He took pyknic from Greek puknos, thick or close-packed. It appeared first in his book Körperbau und Charakter (Physique and Character) in 1921, from where it soon moved into English.
The American psychologist William Sheldon built on and modified Kretschmer’s ideas, coming up with the three terms to describe body types — endomorph, mesomorph and ectomorph — that are now more common. He considered Kretschmer’s pyknic type to be a mixture of the endomorph (with a soft round body tending to put on fat, the Santa Claus type) and the mesomorph (with a compact, powerful, and athletic body tending to the Tarzan or Mr Universe type) and came up with the phrase pyknic practical joke to describe a person who is muscular in early life but who later goes pear-shaped and balloons out into obesity.