The Official Definition: A fictional character who originally appeared in Rudyard Kipling's short story "In the Rukh" and then went on to become the most prominent and memorable character in his fantasies, The Jungle Book and The Second Jungle Book.
My Definition: A young child who is in a state of wildness. This can be seen in untidy hair, mussed clothing, and/or bare, dirty feet. This state of being is usually seen in younsters at the end of the day or after playing outside.
Note the ill-fitting shirt, bad hair, and wild eyes
In this case, complete lack of clothing and unstyled hair a Mowgli makes
The term actually originated when my (much younger) cousin Tasha was little. When she'd run around my grandma's house playing, looking so wild and free, she reminded us of Mowgli from the Jungle Book. The term just stuck around after that, being used for my sisters' children and eventually mine too. So, if you ever hear me refer to a child as Mowgli or Mowgli-ish, now you know why.