What is a spoonerism?

A spoonerism is the interchange of the initial sounds of two words, for example, “he is a boiled sprat”. It is an error in speech or deliberate play on words in which corresponding consonants, vowels, or morphemes are switched. It is named after the Reverend William Archibald Spooner, warden of New College, Oxford, who was notoriously prone to this tendency.