From http://www.netpower.no/~ranveig/piethein/grook.html

A grook, or "gruk" which is the original Danish name, is a short, often humorous poem, which expresses some sort of wisdom. Piet Hein invented the grooks during World War II, when Denmark was occupied by Germany. The short poems were easy to remember, and had a hidden meaning that wasn't easily understood by the Germans and their helpers. The name of these poems, as well as the signature used, "Kumbel", were picked simply because nothing else could be associated with them.

Odud