There’s a cool trick to forming adverbes that lies in the history of la langue française. This trick doesn’t apply to all adverbes, only those that end in –ment.

Franchement
Joyeusement
Seulement
Artificiellement

It’s easy to see how these adverbes are formed with some root and then add –ment to them. What’s less obvious is why you have to say franchement and not francment.

The answer lies in the etymology of the word, going all the way back to Latin. Au début (at first), these words weren’t adverbes, but une combinaison (a combination) of un nom (a noun) and un adjectif (an adjective). You just have to get into the right state of mind to understand it!

The suffix –ment vient du mot latin (comes from the Latin word) MENS, meaning mind or spirit, and can still be seen in words like mentalité (mentality), mental, mentor, etc. The entire history from MENS to –ment is a bit more complicated, but for now just keep MENS in mind.

What’s important for our adverbe story is that in Latin, MENS is a feminine noun. It was used to create something that resembles an adverbe, with the construction adjective MENS meaning dans l’esprit d’adjectif (in the spirit of adjective).

La question est donc (The question is then), if this all comes from Latin, why is the adjective feminine in Latin? The answer is simply that Latin had gendered nouns like French does and MENS, being a feminine noun, needed a feminine adjective to go along with it. La même chose (the same thing) continues in modern French with its mental adverbes.

And voilà:

Franc – Franche – Franchement
Joyeux – Joyeuse – Joyeusement
Seul – Seule – Seulement
Artificiel – Artificielle – Artificiellement