Reversal of the initial letters or syllables of two or more words, such as "I have a half-warmed fish in my mind" (for "half-formed wish") and "a blushing crow" ("a crushing blow").
The word is derived from the name of William Archibald Spooner (1844–1930), a distinguished Anglican clergyman and warden of New College, Oxford, a nervous man who committed many "spoonerisms." Such transpositions are often made intentionally for comic effect.