noun
Refers specifically to the sound produced by blowing air through pursed lips, not a mechanical whistle.
彼は嬉しそうに口笛を吹いた。
He whistled happily.
口笛でメロディーを奏でる。
Play a melody by whistling.
Standard kanji spelling; high priority.
Mixed kanji-kana spelling; less common but still encountered.
笛 is a general term for any flute or whistle, including instruments. 口笛 specifically means whistling with the mouth.
Compound of 口 (mouth) and 笛 (whistle/flute). The exact historical derivation is straightforward; the word has been used for lip whistling since at least the Edo period.