From Vulgar Latin *disiēiūnāre 'to break the fast', Old French disner became Middle English diner — making 'dinner' an etymological twin of 'breakfast', both descended from the concept of ending a fast.
The main meal of the day, taken either at midday or in the evening depending on cultural and regional convention.
From Old French disner (to dine, to eat the first meal), from Vulgar Latin *disiēiūnāre, meaning 'to break the fast' — a compound of dis- (un-, breaking) and Latin iēiūnium (fast, abstinence from food). This makes 'dinner' an etymological sibling of 'breakfast': both words mean 'breaking the fast', but they entered English through different routes and at different times. The Old French disner originally referred to the first meal of the day, not the last