Merde
Merde means “shit” in French.
It is the most common French swear word.
Like the word “shit” in English, merde can refer to the actual substance, or be used in a metaphorical way.
Note that while merde is mostly used as a swear word, in the theater, it’s the equivalent of “Break a leg” – that is, a term actors say before a show, instead of “Good luck”.