Placoso is slang, especially in parts of Mexico, for someone who looks tough, intimidating, or gangster like, carrying a certain street credibility just through their appearance.
In "Se mira bien placoso con el R muy terciado" ("He looks very tough with the R slung across him"), the word describes someone whose whole look, including the rifle slung over his shoulder, screams danger.
"El Chinito (Version Tololoche)" is a corrido about a young man from Southern California nicknamed "El Chinito" (also called "El Gordito" or "El Sobrino"), known for his sleepy, half-closed eyes from smoking marijuana. Set to a tololoche (upright bass) arrangement, the song traces his life from roaming the streets as a kid to finding his footing after moving north, now recognized for his nice cars and easygoing lifestyle. Throughout, it celebrates rolling and smoking marijuana as a constant ritual, naming different strains and forms. Near the end he says goodbye content, dedicating the song to his late father, who he says still watches over him from above.