Spoonerisms, also known as "metathesis", are a specific form of wordplay involving the swapping syllables between two words in a phrase, in such a way that the meaning of the phrase is completely altered.

At BadPuns.com, we use the term Spoonerism quite broadly. Strictly speaking, a spoonerism refers specifically to the exchanging of the first syllables of two words, but similar comedic effects can be had with swapping multiple syllables, or middle syllables, or syllables of more than two words. These variants all have their own names - for example, exchanging the middle syllables of two words is sometimes known as a kniferism - but these names are not in common usage and the distinctions are subtle enough that most people tend to refer to them all as spoonerisms.

The name 'spoonerism' comes from a certain Reverend Spooner, who was a priest and university professor at Oxford, England, and who lived between 1844 and 1930. He suffered from regular tips of the slung (slips of the tongue), and rapidly became famous for them.

See also:

List of Spoonerisms at BadPuns.com
Reverend Spooner's own Spoonerisms
Wikipedia's Spoonerism page (ref Kniferisms and Forkerisms)