Wikipedia
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 high interest or approval of something or someone (originally a young girl or woman thought to be physically/ sexually attractive). Today, in English-speaking countries, wolf-whistling is considered a form of sexual harassment.
According to Adam Edwards of Daily Express, 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 upon seeing a sexy, attractive woman in order 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. Doubt was cast upon this explanation by lexicographer Grant Barrett who noted that it was very thinly supported during a 2015 broadcast of A Way with Words.