in·fan·tile (?n?f?n-t?l?, -t?l)adj.1. Of or relating to infants or infancy.2. Displaying or suggesting a lack of maturity; childish: infantile behavior; an infantile remark. See Synonyms at young.[Middle English infantil, from Latin ?nfant?lis, from ?nf?ns, ?nfant-, infant; see infant.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.infantile (??nf?n?ta?l) adj1. like a child in action or behaviour; childishly immature; puerile2. of, relating to, or characteristic of infants or infancy3. in an early stage of development infantility nCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014in?fan?tile (??n f?n?ta?l, -t?l) adj. 1. characteristic of or befitting an infant; babyish; childish. 2. of or pertaining to infants or infancy. 3. youthful (def. 4). [1690?1700;