Déter' is a colloquial, shortened form of the French word "déterminé," meaning "determined." This kind of informal shortening is very common in spoken French and particularly in rap lyrics, making it an interesting word for learners.
In the song, the artist uses it to assert his resolve and strength, saying, "Que j'suis le plus déter'" (That I'm the most determined). It gives a glimpse into authentic, everyday French slang.
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