The correct spelling of the phrase "letted one hair down" is actually "let one's hair down." The pronunciation of this phrase can be transcribed as /lɛt wʌnz hɛr daʊn/. The spelling may seem confusing due to the archaic past tense form of "let" being "letted" instead of "let." However, this phrase has evolved to become an idiomatic expression meaning to relax or be oneself in a comfortable setting. So, next time you want to let your hair down, remember the correct spelling!