Tch is a slang term, a shortened and informal way of saying "tchoine," which is a derogatory term for a woman, similar to "bitch" or "whore" in English.
In the context of the song, the artist uses this word to express his frustration and anger towards a woman he feels has wronged him, highlighting a moment of intense emotion and conflict in the lyrics.
SDM tells the story of a relationship he knew was doomed from the first line: "Faudra d'la foi et du courage" (It'll take faith and courage). She wants his name for herself, wants him to drop his friends, wants the new Courrèges collection. He even jokes about his own nickname, Ocho, admitting "j'étais le huitième de tous ses mecs" (I was the eighth of all her guys). Every time he decides to leave, she knows exactly how to pull him back, and he confesses it was only his mouth that ever said no.
The title is the punchline. When he tells her he can't live without her, her condition is "si tu m'payes une Cartier Santos" (if you buy me a Cartier Santos), the square luxury watch. He wishes her well at the end, and she answers "souhaite-moi la richesse" (wish me wealth) and swears at health. Brutal, and a little funny, because he still sounds in love.