what word category does spoonerism belong to

Slang/spoonerism for "brain damage"

A "spoonerism" is attributed to Rev. William Archibald Spooner; 1900: http://everything2.com/title/Spoonerism ;)

Spoonerism is a noun, derived from the name of Reverend William Archibald Spooner, known for mixing up the initial sounds of words unintentionally.

What it means when you speak a spoonerism is you are changing the letters around in the sentence to make light of a subject or when you are making a mistake.

Spoonerism is a mistake in a person's speech where vowels, consonants, and morphemes are switched. These are often heard as slips of the tongue.