The first syllable is pronounced like the word “pure” and the second rhymes with the words “file”, “pile” and “mile”. The main stress is on the first syllable. The word has a negative connotation.
If you refer to someone as being “puerile”, you mean that the individual is being silly or childish. The word can also be used to talk about an individual’s action.
*The newspapers said that the plot of the new movie was puerile. *The participants didn’t laugh at my joke. They said it was puerile.