Cul-de-sac is another funny French phrase. “Butt of a bag,” meaning “bottom of the bag,” is the literal translation: cul (butt) de (of a) sac (bag), though the original meaning has nothing to do with how English or French speakers actually use the phrase.
Is it all coming back to you now?
Cul-de-sac, mostly seen in real-estate writing, is an elegant English way to say the “dead end” of a road. In French, the elegant, more formal way to say “dead end” is “impasse,” not cul-de-sac.
Though we try to be, now we know the phrase cul-de-sac is not truly elegant whatsoever, which is a just one of the many created by the French with the word “cul” (butt).
Cette rue a mené à un cul-de-sac. (This road has lead to a dead end.) – Informal
Nous sommes arrivé à une impasse. (We have arrived to a dead end.) – Formal