WHISTLE, the shrill warbling sound made by forcing the breath through the lips, contracted to form a small orifice, or produced by means of an instrument of the whistle type; also, generally, any similar shrill, hissing or warbling sound, as of a bird's note, of wind through trees, ropes, &c. The O. Eng. hwistlian, to whistle, and hwistlere, whistler, piper, are closely allied to hwisprian or hwœstrian, to whisper, to speak softly or under one's breath; and both are imitative words, representing a shrill hissing sound, cf. Ger. wispeln, to whisper, Dan. hvisle, to whistle. The instrument known as a “whistle” takes many forms, from the straight flute and flageolet type made of wood or metal and pierced with holes, to the metal signalling pipe used for signalling on board ship or by policemen. Similarly the term is used of the instruments sounded by the escape of steam on a locomotive or other engine and on steamships, &c., as a means of giving signals.