McCartney wrote the song when he was on holiday. He explained in a YouTube video
that he "was just noodling around on my guitar, thinking about the days when I had a lot of free time."
He remembered how he "used to just lie around doing nothing all day, getting a bit stoned... busy doing nothing!" So, McCartney came up with a little riff on his guitar and the words for the chorus came to him. "It's a song about growing up," he explained. "You know, there's a period in your life, or in some people's lives, when you're not being as productive or as organized or as disciplined as you may later turn out to be."