This is the 5830th most frequent French word.
cosigné
"Cosigné" means "co-signed" in English.
The word "cosigné" is used here as a past participle to indicate that the subject (I) shared the signing responsibility of the agreement.
J'ai cosigné l'accord avec mon partenaire de projet.
I co-signed the agreement with my project partner.
In this sentence, "cosigner" is the infinitive form of the verb, showing the action of sharing the signing responsibility.
Elle m'a demandé de cosigner le document important.
She asked me to co-sign the important document.
Here, "cosigné" is again the past participle, indicating that a group shared the responsibility of signing multiple letters.