spoon·er·ism 
(spnə-rĭz′əm)
(spnə-rĭz′əm)n.
A transposition of sounds of two or more words, especially a ludicrous one, such as Let me sew you to your sheet for Let me show you to your seat.
[After William Archibald Spooner (1844-1930), British cleric and scholar.]