Té-cô is a fascinating example of French slang, specifically "Verlan," which is a type of argot where syllables of a word are inverted. It comes from the word "côté" (side).
In the song, Franglish sings, "J'mets ma fierté de té-cô," which literally means "I put my pride to the side." This phrase shows him setting aside his ego and pride for the sake of love, making it a very engaging and culturally rich word to learn.
Franglish's "Où T'étais" is a super fun and flirty track where he's basically saying, "Hey, where have you been all my life?" He's got this playful confidence, admitting he has an ego but also that he's completely smitten. He sees this woman and immediately gets "des idées" [ideas] and knows she's into him, saying, "Te fais pas désirer je sais qu'j'te plais" [Don't play hard to get, I know you like me].
It's all about that moment when someone finally falls into your arms after you've been looking for them. He repeats, "Hello où t'étais? Je t'ai cherchée te voilà" [Hello, where were you? I looked for you, here you are], like he's been waiting for her. He even admits, "J'ai peur, son cœur j'ai volé" [I'm scared, I stole her heart], but quickly follows up with "Faut même pas s'affoler / Avec moi tout est carré" [No need to panic / With me everything's alright]. He's ready to put his pride aside, declaring, "J'mets ma fierté de té-cô / Pour te dire qu'j'veux pas / J'veux pas te voir partir" [I put my pride to the side / To tell you that I don't want / I don't want to see you leave]. It's a sweet, catchy declaration of finding that special person and not wanting to let them go!