/bentau/ → /bentɔː/ → /bentoː/. Probably a variant of 面(めん)筒(とう) (mentō, “(obsolete) tub for distributing portions of food”), from 面(めん) (men, “face”) + 筒(とう) (tō, “bucket, tub”) (originally referring to a tub for washing the face), both from Middle Chinese. The use of 弁 for ben is an example of phonetic ateji (当(あ)て字(じ)), due to 便 having another sense of “human excrement”.