from The American Heritage® Dictionary of the English Language, 5th Edition.
- noun Childish behavior in an adult, especially as a symptom of mental illness.
Sorry, no etymologies found.
Instead of playing with the freedom and intensity of children, they play with the "blend of adolescence and barbarity" that Huizinga calls puerilism, investing games with patriotic and martial fervor while treating serious pursuits as if they were games.
Such reactions, though occasionally given by normal subjects, are more often given by insane ones, and seem to be somewhat characteristic of states of mental deterioration which are clinically rather loosely described as puerilism.