1) an infant, little child
2) a minor, not of age
3) metaphorically childish, untaught, unskilled
Part of Speech: adjective
Citing in TDNT: 4:912, 631
From an obsolete particle νη ne; implying negation and G2031; not speaking, that is, an infant (minor); figuratively a simple minded person, an immature Christian: - babe, child (+ -ish).
These two sites give similar information, with the definition from several dictionaries and statistics on the use of the word.
Perseus Digital Library
University of Chicago's Logion lexicon
Thayer's dictionary plus other information.
From this site's dictionary (in Italian)
Click on the first column to search for that word as a form of the root νήπιος; click on the second column to search for that grammatical form of the root νήπιος; click on the third column to search for that word and grammatical form; click on the total to search for the root νήπιος.