Doctorado literally means "doctorate" or "PhD". It's an interesting and somewhat unexpected word to find in a reggaeton song, making it stand out.
In "Lo Peor", Nicky Jam uses it metaphorically to emphasize his expertise in a negative trait. He sings, "Pero tengo un doctorado en mentir" (But I have a doctorate in lying), implying he's a master or an expert at deception. This clever use of a formal academic term in a casual context adds a touch of dark humor and makes the lyric memorable.
Nicky Jam and Chris Lebrón spend the whole song on one uncomfortable admission: the person walking out is the one who would have done the damage. It opens on "Hoy te digo adiós, y no es lo que quiero" (Today I say goodbye, and it's not what I want), and then the reason arrives with no cushioning. "Pa' romper promesas soy el mejor" (When it comes to breaking promises I'm the best), so "Si te quedaste, tocará lo peor" (If you stay, the worst is coming). It is a breakup song where the singer casts himself as the thing you should be glad to escape.
The bluntest line is also the funniest. "Pero tengo un doctorado en mentir" (But I've got a PhD in lying) sounds like a joke he isn't really laughing at, and the next two lines drop the joke completely: "Aprendí muy bien a herir" (I learned real well how to hurt), "Con cosas, cosas que nadie se espera" (With things, things nobody expects). Around all of it he keeps trying to make the exit sound merciful. "Qué sabio es el tiempo, y lo sabes" (Time is so wise, and you know it) is the kind of thing you say when leaving early is the only decent move you have left.