sup(夕食を食べる)、-er(名詞の接尾辞)から。
Formerly, the last of the three meals of the day (breakfast, dinner, and supper); now applied to the last substantial meal of the day when dinner is taken in the middle of the day, or to a late meal following an early evening dinner. Supper is usually a less formal meal than late dinner. [OED]Applied since c. 1300 to the last meal of Christ.