Real Dictionary

spoonerism
countable noun: A spoonerism is a mistake made by a speaker in which the first sounds of two words are changed over, often with a humorous result, for example when someone says ' wrong load' instead of 'long road'.

spoonerism in American English
an unintentional interchange of sounds, usually initial sounds, in two or more words (Ex.: “a well-boiled icicle” for “a well-oiled bicycle”)
noun: the transposition of initial or other sounds of words, usually by accident, as in a blushing crow for a crushing blow

spoonerism in British English
noun: the transposition of the initial consonants or consonant clusters of a pair of words, often resulting in an amusing ambiguity of meaning, such as hush my brat for brush my hat