Glistering, exceeding white (stilbonta leuka lian). Old words, all of them. Mat 17:2 has white as the light (leuka hōs to phōs), Luk 9:29 “white and dazzling” (leukos exastraptōn) like lightning.
So as no fuller on earth can whiten them (hoia gnapheus epi tēs gēs ou dunatai houtōs leukānai). Gnaphō is an old word to card wool. Note houtōs, so, so white. Some manuscripts in Matthew add hōs chiōn, as snow. Probably the snow-capped summit of Hermon was visible on this very night. See note on Mat 17:2 for “transfigured.”