Formidable means "wonderful" or "great", but the genius of this song is in its wordplay. Stromae masterfully breaks the word in two to create a hidden, opposite meaning.
He sings, "Tu étais formidable, j'étais fort minable" (You were wonderful, I was very pathetic). This clever pun, where the single word formidable sounds just like the phrase fort minable, captures the entire song's theme of feeling worthless after a relationship that once felt so great.
This song is one big, messy, public breakup. Stromae plays a character who has just been dumped and is now completely drunk, stumbling around and talking to anyone who will listen. The whole song is built on a clever French pun in the chorus: "Tu étais formidable, j'étais fort minable" (You were wonderful, I was very pathetic). He flips between remembering how great his relationship was and how pathetic he feels now that it's over.
He's not just sad, he's angry and wants to make everyone else miserable too. He hits on a woman, then immediately tells her he's been single since yesterday. He tells a random married guy that his wife will eventually leave him. He even gives a poor kid some brutally cynical life advice: "Si Papa trompe Maman / C'est parce que Maman vieillit" (If Dad cheats on Mom / It's because Mom is getting old). It's a hilarious and uncomfortable look at someone hitting rock bottom, feeling like everyone is just staring at him "comme un singe" (like a monkey).