First, Second, THRID?

A friend affected a twang today and told me I looked "right purty". Researching purty led to the discovery that once upon a time, a hros could irnan a race and come in thrid.

metathesis
noun
(Linguistics) Transposition within a word of letters, sounds, or syllables, as in the change from Old English brid to modern English bird or in the confusion of modren for modern.
[Late Latin, from Greek, from metatithenai, to transpose : meta-, meta- + tithenai, to place.]
"metathesis." The American Heritage® Dictionary of the English Language, Fourth Edition. Houghton Mifflin Company, 2004. Answers.com 25 Oct. 2005. http://www.answers.com/topic/metathesis

The process has shaped many English words historically. Bird in English was once bryd, run was once irnan, horse was hros, wasp is also recorded as wæps and hasp, hæps. The discrepancy between the spelling of iron and the usual pronunciation is the result of metathesis.
"metathesis." Wikipedia. Wikipedia, 2005. Answers.com 25 Oct. 2005. http://www.answers.com/topic/metathesis

For example, the word third used to be thrid, and bird, brid. By the same process, English pretty often came to be realized as purty in regional speech. Most such words stabilized because of the influence of printing and the resultant standardized spelling, but purty for pretty has survived in regional American dialects.
"purty." The American Heritage® Dictionary of the English Language, Fourth Edition. Houghton Mifflin Company, 2004. Answers.com 25 Oct. 2005. http://www.answers.com/topic/purty