Hazmerreír literally translates to "make me laugh," but it's used as a noun meaning "laughingstock" or "butt of a joke." It's a vivid and somewhat aggressive term that isn't commonly found in every song.
In the context of this song, Nyno Vargas uses it to criticize those who try to gain fame by imitating others, ultimately becoming a joke to the city: "Siendo el hazmerreír de to' la ciudad" (Being the laughingstock of the whole city). This word perfectly captures the song's critical and street-smart tone.
Nyno Vargas introduces himself as "el Nyno, el más sonao' de tu barrio" [the most famous guy in your neighborhood], a local idol who gets a lot of criticism. He thinks it's because he has something others don't. He then describes a certain type of girl, "la típica choni que se maquilla para comprar el pan" [the typical 'choni' who puts on makeup to buy bread], who acts innocent but is a "arma letal" [lethal weapon] in bed. He also talks about girls who brag about their many partners, saying "me he tirado a 15 y aún no tengo los 20" [I've slept with 15 and I'm not even 20].
The song also takes aim at fakers and wannabes. Nyno says everyone wants fame and to sound street, with money and women, and they all try to copy his style. He warns imitators, "Antes de ser mi enemigo, recuerda que eras mi fan" [Before being my enemy, remember you were my fan]. He calls out guys who spend all day at the gym but are still ugly, and criticizes those who just want to "aparentar" [pretend] and "vivir del cuento" [live off a story], becoming the laughingstock of the city. He tells them to retire because "la envidia es lo que te quema" [envy is what burns you].