"Boojum Snark" is a reference to Lewis Carroll's "The Hunting of the Snark." The poem ends with "For the Snark was a Boojum, you see."
To appease Opie and seal the blood-brother pact by the campfire, Andy's speech is as follows: "In the name of Boojum Snark, spirit of the fire and Brillin Trant, spirit of the water and Grovely Barch, spirit of the air, I hereby declare us blood-brothers, never to be separated for ever and ever."
Although this is the final episode in which Peggy appeared, it's obvious that it was filmed earlier because in previous episodes Peg and Andy were already dating and Opie had a good relationship with her.