infantine comes from French infantin, from Latin infans ('infant'), from in- ('not') + fans, present participle of fari ('to speak'). Morphologically, the prefixed shape matters: the initial element belongs to the recorded formation and helps explain the modern form's surface analysis. In present English, the adjective sense is "of or like an infant; childish or babyish", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as infant, infantile, and infancy. Its semantic neighborhood includes infantile, childish, babyish, and juvenile, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Latin infans and Latin fari.