Engruese comes from the verb engrosar, which means "to thicken" or "to fatten up." In a literal sense, it refers to making something larger or more substantial.
In the context of this song, Dharius uses it in a confrontational way, telling his rival to "keep fattening up" or "keep bulking up" (SÃgale de engruese). It's a taunt, implying that the rival is getting ready for a fight or a challenge, and Dharius is ready to take him on. It's a less common, more aggressive term than simply saying "get bigger," making it quite memorable.
Dharius kicks off "MSD" by waking up, smoking, and showering, only to find out someone's been talking trash about him in a song. He listens to it and finds it pretty funny, especially since the diss track claimed to reveal secrets, but Dharius says he's already told everyone everything. He even calls out the other artist, Renato, saying his style is weak and that he's been waiting for a chance to get back at him.
Dharius doesn't hold back, calling Renato a "niño rata" [rat kid] and making fun of him for liking horchata, but then quickly clarifies that he likes horchata too, just with "morras de mi forje" [girls from my crew], unlike Renato, who he accuses of being with underage girls. Dharius boasts about partying hard with lots of beer and chaos, while Renato is just "ladre y ladre" [barking and barking]. The song ends with Dharius feeling victorious after nine hours, ready to film a video, and mocking Renato for taking seven days to write a bad song, calling him a "millonario sin dinero" [millionaire without money] and a "marrana de mierda" [shit pig].