spoonerism \SPOO-nuh-riz-uhm\, noun:
The transposition of usually initial sounds in a pair of words.

Some examples:
We all know what it is to have a half-warmed fish [“half-formed wish”] inside us.
A well-boiled icicle [“well-oiled bicycle”].
It is kisstomary to cuss [“customary to kiss”] the bride.
Is the bean dizzy [“dean busy”]?
When the boys come back from France, we’ll have the hags flung out [“flags hung out”]!
Let me sew you to your sheet [“show you to your seat”].

Spoonerism comes from the name of the Rev. William Archibald Spooner (1844-1930), a kindly but nervous Anglican clergyman and educationalist. All the above examples were committed by (or attributed to) him.