lethealgia
n.— «Which I think turned out quite nicely—lethealgia is a neologism I coined, derived from the Greek Lethe, the River of Oblivion in Hades, which is sometimes used to indicate forgetfulness, and algia from the Greek word algos, pain.…Therefore (at least according to me) we now have lethealgia, the unpleasant feeling associated with having forgotten something.» —“Lethealgia Relief!” by Terry Oglesby Possumblog (Trussville, Alabama) May 5, 2006. (source: Double-Tongued Dictionary)