For example, let's take the word cul-de-sac. We even use it in English. When I was little, my friend R lived on the cul-de-sac of our block. In English we pronounce it, kull de sack. jolie.
In French, the word cul means butt (or a** to be frank). De means of; sac means bag or pouch. So quite literally, a cul-de-sac is the "butt of the bag" or more nicely put, the "bottom of the bag". It is a dead end. But I so love the fact that the French have decided that using the word cul is a better choice to describe this idea.
Now let's try déjeuner. When you are à jeun it means that you have an empty stomach. When you have your blood drawn for testing, you do it à jeun. When you wake up in the morning, you are à jeun because you haven't eaten all night. So when you dé-jeuner, you "undo" the jeun...you eat. In French, déjeuner actually means "lunch" - a big meal in the middle of the day. So of course, for breakfast it's quite simple - you have a petit-déjeuner - a small lunch.
Here was my small undoing of the jeun this morning -
Chocolatines (or pain au chocolat if you are not from the Sud-Ouest), baguette, OJ and coffee
and the very Parisian view out my window...another Parisian window