Spoonerism
Etymology
Named after the Reverend William Archibald Spooner (1844–1930), an Oxford don famed — largely by legend — for such transpositions, as in "you have hissed all my mystery lectures." Most examples attributed to him were invented by delighted students, but his name stuck to the phenomenon and became its permanent label.
noun · /SPOO-nuh-riz-um/
A slip in which the initial sounds of two words are swapped, often comically.
What it names
The tongue-tangle of "a blushing crow" for "a crushing blow" — sounds jumping across words, a glitch in speech production that happens to be funny.
Example
He fumbled a spoonerism at the toast — "let us raise a toast to our queer old dean" — and the room dissolved.
Related idea
Malapropism substitutes an entirely wrong word for a similar one; a spoonerism keeps the right words but scrambles the sounds between them.