Superlative form of childly; the most childlike or most characteristic of a child.
From 'childly' + '-est' (superlative suffix). This archaic form would have been used to express the highest degree of a childlike quality, paralleling 'happiest' or 'fastest.'
The superlative 'childliest' is so archaic it barely appears in modern English—we'd simply say 'most childlike'—yet its existence shows that '-est' endings were once much more productive in English, even on unusual adjectives.
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