mid-13c., soper, "the last meal of the day," from Old French soper "evening meal," noun use of infinitive soper "to eat the evening meal," which is of Germanic origin (see sup (v.1)).
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.