The story
From Spooner + -ism, named after Oxford don Reverend W. A. Spooner (1844–1930), who is supposed to have habitually made such slip-ups.
Etymology from Wiktionary, CC BY-SA 4.0.
spoonerism
Today it means: A play on words on a phrase in which the initial (usually consonantal) sounds of two or more of the main words are transposed. Full definition →