The summer I waitressed in Paris I often went dancing in Caveau de la Huchette, which is a very tight venue.
I was sitting out a song, and sitting next to a French lady. A man came before her and asked her to dance. She said, without batting an eyelid, “Non merci”. It was not, “Non, merci”, it was “Non-merci”: all one word and nothing else – no apologetic expression and certainly no excuse.