Cobardes translates to "cowards". It's a strong and provocative word that stands out in a song about a complicated breakup.
In the lyrics, the singer is struggling to move on from an ex who is too kind. He desperately asks his ex to "Haz las cosas que hacen los cobardes" (Do the things that cowards do) and "trátame mal" (treat me badly) so he can finally hate them and forget them. This ironic use of cobardes makes it particularly memorable and enticing.
Have you ever tried to get over someone, only to run into them by complete chance? That's the whole story of this song. The singer is just trying to live their life in a huge city when they unexpectedly see their ex. All it takes is one smile to bring back all the feelings they had carefully locked away "con candado en un cajón" [with a lock in a drawer]. It's that frustrating moment when you realize you haven't moved on at all.
Instead of wanting to be friends, the singer has a very direct and almost funny request. They call their ex "el peor amor que he conocido / Tan perfecto que no te olvido" [the worst love I've ever known / so perfect that I can't forget you]. Because the ex is too nice and perfect, the singer can't hate them. So, they beg the ex to misbehave: "Sé un ex de verdad y trátame mal" [Be a real ex and treat me badly]. They literally need their ex to be a terrible person so they can finally have a reason to get over them.