It was during Toby Keith and Clint Eastwood's first-ever golf tournament together that the country star asked his friend about his ongoing inspiration. "I [asked], 'What keeps you going at your age?'" Keith shared during an interview with the "Superstar Power Hour" podcast. "He said, 'I try to get up every day and be productive — not let that old man in.'"

Throughout the three-day golf tournament, Eastwood also let his friends in on the plot of his upcoming movie "The Mule," which served as further inspiration for Keith's song. This film, both directed by and starring Clint Eastwood, follows a 90-year-old veteran turned drug cartel courier. "I didn't ask him if I could put a song [in the movie]," Keith said during his press conference. "I just went home and wrote it, sent it to him, and hoped that he would consider it. And, you know, he did."

The song, which contains poignant lines like "And I knew all of my life / That someday it would end / Get up and go outside / Don't let the old man in," is featured during the end credits of "The Mule." The moving tune is significant to his friendship with Eastwood, but it also became something of a personal anthem to the late country music star.