whistle

Concise Oxford English Dictionary © 2008 Oxford University Press:
whistle/ˈwɪsl/

▶noun

• 1 a clear, high-pitched sound made by forcing breath through a small hole between partly closed lips, or between one's teeth.

  ■ any similar sound.

• 2 an instrument used to produce such a sound, especially for giving a signal.

• 3 Brit. informal a suit. [from rhyming sl. whistle and flute.]

▶verb

• 1 emit or produce a whistle.

  ■ produce (a tune) by whistling.

  ■ move rapidly through the air or a narrow opening with a whistling sound.

• 2 blow a whistle.

• 3 (whistle for) wish for or expect (something) in vain.

– phrases
blow the whistle on informal bring (an illicit activity) to an end by informing on the person responsible.
(as) clean as a whistle extremely clean or clear.
whistle something down the wind

• 1 let go or abandon something.

• 2 archaic turn a trained hawk loose by casting it off with the wind.

whistle in the dark pretend to be unafraid.

– derivatives
whistler noun.

– origin OE (h)wistlian (v.), (h)wistle (n.), of Gmc origin; imitative.