puer

Puer, pueri, m., second declension, and one of the small group of -er nouns that keep the e all the way through: pueri, puero, puerum, not the pattern of ager, agri or magister, magistri where the e drops. The vocative is puer, unchanged, since the -us of the ordinary vocative rule never existed here.

Two things to watch. Puer is not strictly male in older Latin: it can mean a child of either sex, with puella as the specific feminine, and the diminutive puerulus and the abstract pueritia, "boyhood", both belong here. And it does the same social work English "boy" once did, meaning a slave or a serving lad; when a character in Roman comedy bellows puer at the door, nobody is calling for a schoolchild.