Today's word is PUER, which is a second-declension masculine noun (gen. pueri).
Latin meaning and usage: The basic meaning of the word puer is "boy." It referred to a young man up to the age of seventeen, and could also be applied to someone even older. In archaic Latin, it originally referred to a "child," but later the word became restricted to a male child or boy, while the word puella referred to a girl.
Latin word formation: There is an adjectival form, puerilis, with further derivatives: pueriliter (adverb) and puerilitas (noun). The word pueritia means "childhood" or "boyhood." The adjective puerpera refers to a woman in labor, about to deliver a child. There is a diminutive: puerulus.
English cognates and derivatives: In English we use the word "puerile," which originally meant in English something "youthful" or "boyish," but which now has negative connotations of being "childish."
Here are some examples of today's word in Latin sayings and proverbs; for more examples, see the page at the Scala Sapientiae, which also contains notes on some of the proverbs cited below:
Aliud legunt pueri, aliud viri, aliud senes.
Quod puer non didicit, non discet vir.
Quod puer non didiceris, seni tibi discendum erit.
Ea pueri discant quibus sunt senes usuri.
Ne puero gladium.
Puero gladium ne committas.
Fabulae decent pueros.
Disce, puer, tenero dum flos tibi floret in aevo.
Bis puer senex.
Stultus puerque vera dicunt.
Melior est puer pauper et sapiens rege sene et stulto.
Erudiendi pueri in aetate tenera.
Saepe sibi proprium fecit puer ipse flagellum.
Indulge veniam pueris.
Est pueris carus qui non est doctor amarus.
Vae terrae, cuius rex puer est.
Non puero cultellum.
Etiam puer et stultus opportuna loquuntur.
Puer, sacer est locus; extra mingito.
Peccantem puerum quisquis non corrigit, odit.