Definition of whistle:
(v) :
to make a shrill, high-pitched sound by forcing air through the mouth. To produce a whistling sound, restrictions to the flow of air are created using the teeth, tongue and lips; to move in such a way as to create a whistling sound
"a bullet whistled past."
(n) :
a device designed to be placed in the mouth in order to make a whistling sound; a shrill, high-pitched sound made by whistling