quack
Explanation
The Joke
The comic opens with someone excitedly making duck noises: "Quack! Quack! Quack! I'm a duck! Quack! Quack!" They then address a Catholic cardinal: "Oh! Cardinal Piacenza! How long have you been there?" The cardinal sternly replies: "Your Holiness, let us never speak of this and act as if there is no basis for a Quackius Pater. Duckibus Bill." The Pope (revealed to be the duck impersonator) responds: "Yes, Your Holiness, I concur."
The final panel shows a caption: "This quacking thing really is true after a while, am I right?" followed by "Please remember to lock your door, Your Holiness."
The joke imagines the Pope being caught in an embarrassing private moment -- acting like a duck -- and the ensuing awkward cover-up. The cardinal's response is to immediately propose they pretend it never happened, using mock Latin to make the cover-up sound official and theological.
The Humor
The comedy comes from the contrast between the immense dignity and authority of the papacy and the silly, childlike behavior of quacking like a duck. The use of fake Latin ("Quackius Pater, Duckibus Bill") to formalize the cover-up is a parody of how the Catholic Church uses Latin to lend gravity to its pronouncements -- here applied to the very un-grave matter of the Pope's duck impression. The joke also plays on the idea that powerful institutions have elaborate mechanisms for covering up embarrassing incidents, even ones as harmless as this.
References
Cardinal Piacenza is a real Catholic cardinal (Mauro Piacenza), which adds a layer of specificity to the absurdity. The fake Latin phrases parody actual papal terminology like "Pater" (Father) and other ecclesiastical Latin.