A spoonerism is a speech error in which the speaker switches the initial consonants of two consecutive words. If you say "bunny phone" instead of "funny bone," you've uttered a spoonerism.
"Jelly beans" becomes "belly jeans." "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. The first two examples above, by the way, are modern spoonerisms. The last? A 100% genuine original spoonerism.