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mid-15c., "pertaining to infants," from Latin infantilis "pertaining to an infant," from infans (see infant). Sense of "infant-like" is from 1772.
infantile in·fan·tile (ĭn'fən-tīl', -tĭl)adj.
Of or relating to infants or infancy.
Displaying or suggesting a lack of maturity; extremely childish.