Merecías is the imperfect past tense of the verb merecer, meaning "to deserve." It's a word that carries significant emotional weight, especially in the context of a breakup or a relationship's end.
In this song, the singer repeatedly reflects on the painful realization, "Siempre lo supe, que tú no me merecías" (I always knew that you didn't deserve me) and "que yo no te merecía" (that I didn't deserve you). This word is key to understanding the song's central theme of self-worth and the complex emotions involved in recognizing an unequal or unhealthy relationship.
This is a breakup song built out of the things neither person said while there was still time to say them. The first verse does the accounting: "Los 'te quiero' los guardaste en los bolsillos" ("You kept the 'I love you' in your pockets"), and then "Y los abrazos los colgaste en las paredes" ("And you hung the hugs on the walls"). Affection treated like furniture, kept in the house and never actually handed over. By the end of the verse the silence is doing all the talking: "Y tu silencio me decía 'vete'" ("And your silence told me 'go away'").
Then the duet plays its trick. The chorus comes around a second time with every pronoun flipped. "Cuando agarraba tu sombrero y tu camisa" ("When I grabbed your hat and your shirt") becomes "Cuando agarrabas mi sombrero y mi camisa" ("When you grabbed my hat and my shirt"), and the line that actually stings, "Siempre lo supe, que tú no me merecías" ("I always knew you didn't deserve me"), comes back as "Siempre lo supe, que yo no te merecía" ("I always knew I didn't deserve you"). Same memory, same accusation, two people holding opposite ends of it. The bluntest moment is in the bridge: "¿Pa' qué mentirles? Me dediqué a perderte" ("Why lie to them? I dedicated myself to losing you"). The hat and the shirt are the whole point. Nobody wanders around the house in your clothes once they have stopped loving you.