1) voluntary, arbitrary worship
1a) worship which one prescribes and devises for himself, contrary to the contents and nature of faith which ought to be directed to Christ
1b) said of the misdirected zeal and the practice of ascetics
Part of Speech: noun feminine
Citing in TDNT: 3:155, 337
From G2309 and G2356; voluntary (arbitrary and unwarranted) piety, that is, sanctimony: - will worship.
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 ἐθελοθρησκία; click on the second column to search for that grammatical form of the root ἐθελοθρησκία; click on the third column to search for that word and grammatical form; click on the total to search for the root ἐθελοθρησκία.