According to some accounts, the nickname was first used by Nancy Sinatra, Frank Sinatra's daughter. In an interview, she mentioned how her father's eyes, which appeared more gray, could sometimes look deep blue under the right lighting conditions, giving him an intense and captivating gaze.
Over time, the nickname gained popularity among Sinatra's fans, the media, and colleagues in the entertainment industry. It became synonymous with his charismatic presence, musical talent, and the aura he exuded on and off stage. "Ol' Blue Eyes" became a term of endearment and admiration, representing the essence of Frank Sinatra's iconic stardom.