1) voluntarily, willingly, of one's own accord
1a) to sin wilfully as opposed to sins committed inconsiderately, and from ignorance or from weakness
Part of Speech: adverb
Citing in TDNT: 221,*
Adverb from the same as G1595; voluntarily: - wilfully, willingly.
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 ἑκουσίως; click on the second column to search for that grammatical form of the root ἑκουσίως; click on the third column to search for that word and grammatical form; click on the total to search for the root ἑκουσίως.