puerility
The state, quality, or condition of being childish or puerile.
countable, uncountable
That which is puerile or childish; especially, an expression which is insipid or silly.
countable, uncountable
You treat his opinions (though he never thrusts them on you) about "the Church," and his duty, and the souls of his parishioners, with civil indifference, as much ado about nothing; and his rubrical eccentricities as puerilities.
Where it comes from
From Middle English puerilite, from Middle French puérilité and its etymon Latin puerīlitās, from puerīlis (“childish, juvenile”), from puer (“boy”). By surface analysis, puerile + -ity.
Adapted from the Wiktionary entry puerility (CC BY-SA 4.0).