Here's the official meaning of spoonerism: "The transposition of initial consonants in a pair of words." Zzzzzzzzz. Okay, pretty dull sounding, but in practice actually pretty funny: "My lips are zipped" becomes "my zips are lipped."
"Popcorn" becomes "cop porn." "Son, it is now kisstumary to cuss the bride." You get the idea. We owe the invention of the Spoonerism, or at least its great fame, to a nineteenth-century English reverend named Archibald Spooner, who was famous for mixing up his words. He wasn't a drunk, although drunkenness can sometimes cause spoonerisms, just absent-minded. The first two examples above, by the way, are modern Spoonerisms, too risqué for the good reverend himself. The last? A 100% genuine original Spoonerism.