Il l’a tourné sens dessus-dessous.
ee lah toor-nay sahss day-sew day-soo. Click below to hear this.
He turned it upside down.
You don’t actually care what he turned, do you? I don’t either. Let’s get right to the upside down part. Directional expressions can be a bit problematic in French, as compared to English! (Note that the reverse is also true. English has no corner on logic. Logic is what you are familiar with. The rest tends not to make sense. Thr world would be a better place if we all understood that.)
Le sens can mean sense (as in common sense), sense (as in the five senses), meaning (the sense of a word), or direction (sens unique means one way on a street sign). Yes, you do pronounce the final s.
If dessus means up, above, on top, and dessous means down, under, below, you can see how important pronunciation is. There’s a big difference between u (/ew/) and ou (/oo/).
And have you noticed how the English and the French correspond to each other? Both have the two directional words (up, down; dessus, dessous), and in the same order. In English, however, we can’t talk about the up or the down, so we need to add in the word side to clarify what we are talking about.
I hope that doesn’t turn your notions of French sens dessus-dessous!