Belek is a very interesting and somewhat rare word in French music, as it's a slang term derived from Arabic, meaning "watch out" or "be careful."
In the context of the song, it's used as a warning, particularly when describing a girl as "magnifique" (magnificent) but also "maléfique" (malicious). The repetition of belek emphasizes the danger and the need for caution, making it a memorable and impactful word that adds a unique flavor to the lyrics.
KeBlack opens with a knock at the door: "Toc toc toc, ouvre-moi cette porte" ("Knock knock knock, open this door"), then flips straight into a threat: "Stop stop stop, pose-moi ce couteau" ("Stop stop stop, put that knife down"). From there it's one long lecture to a friend who won't listen, about a girl who vanishes in her little Volvo, meets someone by the town square, and spins stories he swallows whole: "Tout c'qu'elle te dit tu la crois crois crois" ("Everything she tells you, you believe believe believe").
The hook is him giving up on getting through: "Quoi quoi quoi" ("What what what"), "J'comprends rien" ("I don't understand anything"). By the last chorus she's flipped from "magnifique" to "maléfique" (from "gorgeous" to "evil"), the same warning both times: "belek" ("watch out").