Origins
The word 'supper' names a meal after the act of drinking rather than eating. Its ancestry lies in a Proto-Germanic root *sūp- meaning to sip, slurp, or take in liquid — the same root that produced 'soup' in English, 'Suppe' in German, 'soep' in Dutch, and 'soupe' in Old French. The evening meal was called 'supper' because it characteristically involved sipping — broth, pottage, warm liquid — in contrast to the heavier solid food of the day's main meal.
The Old French verb 'soper' (also 'souper') functioned both as a verb (to eat supper) and, by the standard Old French process of nominalisation, as the name of the meal itself. It entered Middle English in the thirteenth century as 'soper' or 'souper,' settling into 'supper' by the fifteenth century through the regular shortening of the unstressed vowel in the second syllable and the assimilation of the final consonants.
To understand 'supper,' you have to understand that 'dinner' meant something different in medieval England than it does today. 'Dinner' (from Old French 'disner,' from Vulgar Latin *disjejunare, 'to break one's fast') was the main meal of the day and was eaten at midday — typically around noon or in the early afternoon. 'Supper' was the secondary, lighter meal taken in the evening. This distinction was socially encoded: working people ate 'dinner' at midday and 'supper' in the evening; the wealthy or leisured classes might eat 'dinner' later, shifting the whole schedule. The slow movement of 'dinner' from midday to evening across the eighteenth and nineteenth centuries, driven by changing working patterns and social fashion, is one of the great undocumented semantic shifts in English food vocabulary.
Development
The result is that in modern British and American English, 'dinner' and 'supper' have traded places or merged in complex ways. For many British speakers, 'dinner' is now the evening meal, 'lunch' is the midday meal, and 'supper' is a late or informal version of the evening meal. In parts of the American Midwest and South, 'supper' is the standard word for the evening meal, with 'dinner' reserved for a formal occasion. In Scottish and Northern Irish usage, 'dinner' often still means the midday meal. These regional variations are linguistic fossils — each preserving a different stage of the long historical shift.
The Proto-Germanic root *sūp- is likely related to, or possibly the same as, the root behind Latin 'sorbēre' (to sip, absorb — source of English 'absorb'), though the Latin and Germanic forms may be independent onomatopoeic developments from the natural sound of sipping rather than strict cognates. The PIE background is uncertain: a root *seup- or *sub- for sipping is sometimes reconstructed but the evidence is thinner than for core kinship or physical environment terms.
The culinary and social history embedded in 'supper' extends to the most famous supper in Western cultural memory. 'The Last Supper' (in Latin, 'Cena Domini,' the Lord's meal — using a different word, the Latin 'cena') is described in the Gospels as a Passover seder, an evening ritual meal. In translations into English, the word 'supper' was chosen to render 'cena,' giving the scene its familiar name. The word thus carries, in its most famous usage, both its liquid-sipping etymology and the full weight of its social register as an intimate, evening gathering rather than a formal midday feast.