The word "pyknic" is an adjective meaning "short and fat", or an noun meaning "a person who is short and fat". A classic example of a "pyknic physique" would be actor Danny DeVito.
It may sound like this is what happens to a person who goes to too many picnics, but this word is unrelated to those outdoor feasts, and in fact was coined in the early 20th century by German psychologist Ernst Kretschmer, from the Greek pyknos, meaning "thick, dense, concentrated".
Kretschmer was studying the physical forms of German criminals, in an attempt to draw connections between their physiology and their criminal behavior. In his influential 1921 book Körperbau und Charakter (Physique and Character), Kretschmer put forth his theory that tall, thin, weak people, which he called asthenic, were more likely to commit white-collar crime such as fraud or embezzlement, whereas tall, strong, big-boned people, which he called athletic, were, perhaps not surprisingly, more likely to commit violent crime. A third type - people which were neither tall nor strong nor thin - were the pyknic, or short fat people, who committed a mixture of both types of crime.
Kretschmer's theory was very influential for a time, and his book was translated into many languages, which is how the word spread to English. Kretschmer's typology was elaborated upon in the 1940s by American psychologist William Sheldon who recategorized the three body types into endomorph, mesomorph and ectomorph, which are more common in English today.
It was Sheldon who also coined the moderately well-known phrase "pyknic practical joke" to describe a person who begins adulthood as a mesomorph, but ends up a much more pear-shaped endomorph, or "pyknic".