na-adjective, noun
for fun; half in jest
Describes doing something partly for amusement, often with a lighthearted or not entirely serious attitude. Commonly used with に as an adverb.
彼は面白半分にそんなことを言ったのだろう。
He probably said that half in jest.
面白半分で始めた趣味
A hobby I started just for fun has now become my job.