Khapta is a slang term that means "crazy" or "wild," often used to describe a lively or intense atmosphere, or someone acting impulsively.
In the song, PLK says, "J'crois que j'vais mettre khapta, ce soir" (I think I'm going to go wild tonight). It's an interesting word because it's not standard French, but rather a term from North African Arabic slang that has become common in French youth culture, reflecting the diverse influences in modern French music.
"Faut Pas" is PLK's tired, honest complaint about being surrounded by people who don't actually have his back. The hook lays out the rule: don't call yourself my friend if you don't love me, don't promise to show up and then never come.
He opens overwhelmed, his head full of problems, wanting to be left alone while people who don't really know him judge him anyway. He points out that people only pay attention to him because he sells records, not for who he is. Marseille, his city, is going through a rough stretch, and he leans on weed and staying home to cope with it all.
Jul's verse carries the same theme from his own angle: he stayed loyal and low-key all summer, only for people he trusted with his problems to spread them around. His mother's old warning that friends change like the seasons turns out to be true. Underneath the tough talk about knives, cops and money, the song is really about two artists who've learned to be careful about who they call close.