zīdainis

Etymology
From, made into a masculine 2nd-declension noun (ending -is).

Noun

 * 1) infant, baby child in its first year of life, when its basic food is mother's milk
 * 2)  naive person
 * 1)  naive person
 * 1)  naive person
 * 1)  naive person

Usage notes
The feminine form is attested but rather rare.