Bloody vowels!
French has at least two "oo" sounds, and I have considerable and embarrassing difficulty in making them when I speak. A quick example: I casually asked my boss what du coup means, because it's something I've heard said often. At least, that was my intention, but what I actually said was du cul. Du coup means "as a result", while du cul—my amused boss explained—means "from the arse". These words sound frighteningly similar.
As an aside, rude words are really interesting. They normally hold such power, but in a newly learned language they fizzle completely. Suddenly they are just terms; more entries in a long set of words to learn. It's tricky to gauge the relative "badness" of those different words. I first experienced this in a French class in Canberra, when my suave French teacher accidentally demonstrated it to dramatic effect. He mentioned the word con, which is a very mild swear word in French, and then asked if anyone knew what it meant. Met by blank looks he proceeded to translate it brightly to the class, which I might add contained, apart from me, about seven others who were all above the age of fifty. He paused for a beat and then said, forcefully, "it means c**t". There was a very sharp intake of breath in the room, which I am sure our teacher misinterpreted as admiration. Although his translation was anatomically correct, he got the level of the swear word completely wrong, and translated a mild French swear word into practically the worst word in the English language – the one word that Triple J refuses to allow on air, and the one word I feel compelled to censor on this blog. Swear words are tricky and dangerous things in another language.
Anyway, back to the way to say "oo" in French. I've done some reading about French and Australian English phonology on Wikipedia. It is surprisingly interesting.
The two "oo" sounds in question appear in the French words dessus and dessous. Dessus (pronounced d(ə)sy) means "on top of", and the "oo" sound here is what I now know is a close front rounded vowel; it's also in words like su and vu. Meanwhile, dessous (pronounced d(ə)su), means "underneath", and contains a sneaky close back rounded vowel. This noise is found in words like vous, nous, and sous.
The problem for me arises because I default to using the closest sounds from Australian English, which are the near close near back vowel (eg "pool") or the close central rounded vowel (eg "rude"). These are not the same as either of the sounds in dessus and dessous.
So there you have it. Vowels are tricky, and so are swear words, and when a mixup with the former accidentally creates the latter, trouble can ensue. This is why, when you thank someone in French, you must be careful to say merci beaucoup (thanks very much!), and not merci, beau-cul! (thanks, nice-arse!).