Wolf-whistling or finger whistling is a type of whistling in which fingers are inserted in the mouth to produce a louder and more penetrating tone.
A wolf-whistle is a two-toned sound (like 'whip-woo') commonly made using the above technique to show approval of something or someone (originally a person thought to be sexually attractive).
The wolf-whistle originates from the navy General Call made with a boatswain's pipe. The general call is made on a ship to get the attention of all hands for an announcement. Sailors in harbour would whistle the general call when seeing a pretty woman to draw fellow sailors' attention to her. It was eventually picked up by passers-by, not knowing the real meaning of the whistle, and passed on. Over time, the precise tones of the general call were flattened somewhat into the wolf whistle common today.
Although the "wolf-whistle" or "wolf-call" sound can be produced using a conventional whistling technique, a louder and more effective form is usually used. To produce a wolf-whistle, one or more fingers are inserted into the mouth to shape the opening, allowing a forceful stream of air to be blown through.
Generally, the more air forced through the mouth, the louder the whistle.