noun
humming; crooning
Refers to singing softly with closed lips, often unconsciously or while doing something else. Commonly used in the phrase 鼻歌を歌う (to hum a tune).
彼はシャワーを浴びながら鼻歌を歌っていた。
He was humming a tune while taking a shower.
機嫌がいいと、つい鼻歌が出る。
When I'm in a good mood, I end up humming without thinking.
Compound of 鼻 (nose) and 歌 (song). The exact historical derivation is uncertain, but the word naturally describes a song produced through the nose.