Spoonerisms

A spoonerism is an error in speech or deliberate play on words in which corresponding consonants, vowels, or morphemes are switched (see metathesis). It is named after the Reverend William Archibald Spooner (1844–1930), Warden of New College, Oxford, who was notoriously prone to this tendency. It is also known as a marrowsky, after a Polish count who suffered from the same impediment. While spoonerisms are commonly heard as slips of the tongue resulting from unintentionally getting one’s words in a tangle, they can also be used intentionally as a play on words. In some cultures, spoonerisms are used as a rhyme form used in poetry, such as German Schüttelreime. Spoonerisms are commonly used intentionally in humour, especially drunk humour.

http://en.wikipedia.org/wiki/Spoonerism