expression, godan verb (-ku)
General expression for producing a whistling sound with one's mouth.
彼は歩きながら口笛を吹いた。
He whistled while walking.
口笛を吹いて犬を呼んだ。
I whistled to call the dog.
口笛 is the noun 'whistle' (the sound or act), while 口笛を吹く is the verb phrase 'to whistle'.
Compound of 口笛 (くちぶえ, 'mouth whistle') + を (object marker) + 吹く (ふく, 'to blow').