Unfortunately it’s still not available at its old address ageofrocks.org — initial indications are, that’s been grabbed by a domain name squatter. This means that you’ll probably find there are several broken links on the site, but the content is all there.
The American writer Sylvia Wright coined the term in 1954, recalling a childhood memory of her mother reading the Scottish ballad “The Bonny Earl of Murray” (from Thomas Percy’s 1765 book Reliques of Ancient English Poetry), and mishearing the words “layd him on the green” as “Lady Mondegreen”.