The same logic happens with the Portuguese word "desjejum", which can be broken in almost the same manner, meaning "that [meal] which breaks your fast (des+jejum).
Interestingly, in french, it's the same kind of roots (déjeuner, dé- (quite like the un- prefix in english), jeuner (fast)), but the word doesn't actually point to the morning meal, but instead, it points to lunch.
The word for the morning meal is "petit déjeuner", literally "small un-fast". Presumably, people were not having a morning meal a long time ago.