1) to give additional strength, to make stronger
2) to receive greater strength, grow stronger
Part of Speech: verb
From G1909 and G2480; to avail further, that is, (figuratively) insist stoutly: - be the more fierce.
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 ἐπισχύω; click on the second column to search for that grammatical form of the root ἐπισχύω; click on the third column to search for that word and grammatical form; click on the total to search for the root ἐπισχύω.