"To be is to do" — "To do is to be" — "Do-be-do-be-do": what does this mean?

To be is to do. —Socrates
To do is to be. —Plato
Do-be-do-be-do. —Sinatra

I saw it on some shirts or somewhere. I don’t understand what it means when those sentences are put together.

Solution 1:

This is supposed to be humorous.

The first two lines are contemplations on life and are supposed to be very profound. They were spoken by very famous philosophers from Greek history who talk about life and what it is to be alive.

Frank Sinatra was a famous American swing singer around the mid 1900s. The Dobedobedobeo is a vocal sound from music of that era. It is meaningless, a sound used only to help with the harmony of a song or fill in notes when there are no lyrics.

By putting the three together, the last line makes a joke of the first two, either by saying that Frank Sinatra was just as profound as the other two, or that Socrates's and Plato's thoughts were no more deep/important than Sinatra's nonsense line.