naughty
The French word 'méchant' corresponds to 'naughty' in English. It is used to describe someone, often a child, who behaves badly or disobeys rules, similar to the English use. Note that 'méchant' can also translate to 'mean', depending on the context.
He is a wicked man.
This phrase is a straightforward application of 'méchant'. It represents a common use of the word to describe a person who is morally reprehensible or wicked.
This dog is very mean.
In this example, 'méchant' is used to describe an animal's aggressive behavior. So, it can indeed be used to describe not just people, but animals as well.
Do not be mean to your little sister.
Here, 'méchant' is used in a slightly different context. It is an advice given to someone, probably a child, to treat others nicely, in this case, his little sister.