1) born without a mother
2) bereft of a mother
3) born of a base or unknown mother
4) unmotherly, not worthy of the name of mother
Part of Speech: noun masculine
From G1 (as a negative particle) and G3384; motherless, that is, of unknown maternity: - without mother.
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 ἀμήτωρ; click on the second column to search for that grammatical form of the root ἀμήτωρ; click on the third column to search for that word and grammatical form; click on the total to search for the root ἀμήτωρ.