1a) not to have strength, power, or ability, to be weak
1b) can not be done, to be impossible
Part of Speech: verb
Citing in TDNT: 2:284, 186
From G102; to be unable, that is, (passively) impossible: - be impossible.
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 ἀδυνατέω; click on the second column to search for that grammatical form of the root ἀδυνατέω; click on the third column to search for that word and grammatical form; click on the total to search for the root ἀδυνατέω.