A. A. Milne of Winnie-the-Pooh fame contended that light verse is the hardest and most severely technical work known to authorship. If his appraisal be accepted, then unspeakable rhymes, long known as sight rhymes, go beyond the superlatives of difficulty.
Payne, Stanley L.
Word Ways: Vol. 10
, Article 3.
Available at: http://digitalcommons.butler.edu/wordways/vol10/iss2/3