Callow is a synonym of immature used to describe someone, especially a young person, who does not have much experience and does not know how to behave like an adult. Like the word immature, callow is often used disapprovingly.

// The novel’s plot involves a callow youth who eventually learns the value of hard work and self-reliance.

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“Lowery opted to make Gawain a callow young man who aspires to earn the right to join the Knights of the Round Table by proving his honor and bravery—confronting some hard truths about himself along his journey.” — Jennifer Ouellette, Ars Technica, 31 July 2021

Although callow birds—that is, featherless, baby birds—are quite visibly (and audibly) hungry for the world beyond their nest, they are just as visibly immature, far from ready to step, or hop, into it. This meaning of callow isn’t common (we only define the word this way in our Unabridged dictionary), but it both links the word directly to its origin, the Old English word calu, meaning “bald,” and to today’s more common use in describing someone possessed of youthful naiveté. Calu eventually fledged into callow with the same “bald, hairless” meaning, but was applied to bald land too—that is, land denuded of vegetation or not producing it in the first place. By the 16th century, callow had expanded beyond the literal sense of “lacking hair or flora” to its avian use of “lacking feathers” as well as to today’s familiar application to people. Callow now is most often used to suggest the inexperience or immaturity of young people brimming with confidence but still, figuratively, unfledged.