91 whiskey refers to the NATO phonetic alphabet codeword for the letter “W”. It is commonly used in military and aviation communications to avoid confusion when spelling out letters over voice channels.
The NATO phonetic alphabet was developed during World War II to improve communication clarity, especially in noisy or difficult listening conditions. Each letter of the alphabet is assigned a unique word that is easily distinguishable from other words in the alphabet. This helps to reduce errors and misunderstandings when transmitting messages over voice channels, particularly when dealing with unfamiliar accents or languages.