Of unknown origin although this metaplastic word probably came into being as a typographical error. A possible definition: lurid or sensational material that has serious literary, artistic, political or scientific value.
Assumed to be an uncommon coupling of more common misspellings or typographical errors of the words simplest and explanation. Amusing in that simplist is a word and that explination could be a misspelling of explication which is synonymous with explanation. Originally found in a now forgotten book review on amazon.com.