Yakuseki means dinner in Zen sect.
Yakuseki' referred to the acupuncture which was one of the tools that were Biku, Buddhist priests were allowed to carry. This is referred in the description 'the tool to attack diseases is called Yakuseki, and stones were used as acus in old ages while iron were used in modern ages for healing patients.' in the eighteenth version of 'Genno ongi' (annotation of Buddhist terms).
Though the meals in the afternoon were prohibited in the precepts of Buddhism, the meals for medical treatment were referred to 'Yakuseki' because those were allowed.
Obaku shingi' (Obaku Monastic Regulations) shows 'Yakuseki is dinner and Biku is prohibited to eat something in the afternoon, so dinner is called Yakuseki.'
This is for medical treatment for starvation and thirst.'
This is said to be interpreted as dinner based on the spirit of Ritsu (Risshu sect).