Décennie translates to "decade". It's a less common word in everyday French pop songs, making it stand out.
In the song, GIMS uses it to emphasize the intensity of his feelings, singing "Chaque absence dure une décennie, Monica" (Every absence lasts a decade, Monica). This hyperbolic expression conveys how long and painful even a short time apart feels to him, highlighting the depth of his love and longing.
GIMS's "Monica" is a direct and passionate plea to a woman named Monica. He's not messing around, telling her, "Sois pas choqué, j'm'avance vers toi" [Don't be shocked, I'm coming towards you] and asking to touch her shoulder. He just wants a few sweet words to forget her flaws, even if it's just for a moment.
The song is all about his intense desire for her, asking her to "Emmène-moi vers l'infini, Monica" [Take me to infinity, Monica]. He's not talking about a simple trip to Ko Samui; his love is so strong that "Chaque absence dure une décennie, Monica" [Each absence lasts a decade]. He admits to past mistakes, being too focused on making money, but now he's ready to embrace her fully, loving her "comme pas permis" [like crazy].