“Peachy keen” has a long story, beginning in the late 1700’s, when the term peach was used to refer to a Cutie Patootie, or an extremely attractive person.
Keen started to be used as a slang word for fine in the beginning of the 1910’s, especially in spoken contexts, although it also appeared in magazines as well.
“Peachy keen” started appearing in its currently known form in the 1930’s, with one of the earliest examples located in a 1935 issue of the Foreign Policy Association’s Headline Series.
Since the 1930’s, “Peachy keen” has gained significant ground, especially following the 1950’s, appearing in pop culture products, including magazines, television series, movies as well as novels and songs.
It was first defined on Urban Dictionary on November 15th, 2001, with several entries having been written since.
“Peachy Keen” was also used as the title of a 2010 short film, directed by Kevin Mack.