Spoonerism is the shuffling of the first letters of words to make different words and therefore change the actual meaning of the sentence, or else produce a humerous, non-sensical sound. Ex: Rather than "I have to blow my nose", "I have to nose my blows". Or, in an example by Strong Bad, rather than "this is a cellular telophone", "this is a tellular cellophone".
from Wikipedia
Spoonerism is a 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.
While spoonerisms are commonly heard as slips of the tongue resulting from unintentionally
getting one's words in a tangle, they are considered a form of pun when used purposely as a
play on words.
Assisted by Leigh Averett