This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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ad. Willingly, of one's own free will.

ad. Deliberately, on purpose; maliciously.

Of free will or choice; willingly; voluntarily; gladly; readily.

By design; with set purpose; intentionally; especially, in a wilful manner; as following one's own will; selfishly; perversely; obstinately; stubbornly.

In law, wilfully is sometimes interpreted to mean

by an act or an omission done of purpose, with intent to bring about a certain result; or

with implication of evil intent or legal malice, or with absence of reasonable ground for believing the act in question to be lawful.

ad. in a willful manner

"It is not difficult to understand that the painter of a ‘Proserpine’ and a ‘Ghirlandata’ would occasionally feel the luxury of a mood intellectually lazy, and would be minded to give voice to it — as in this instance — in terms wilfully extreme; keeping his mental eye none the less steadily directed to a"

willfully   

skillfully    willfully