Cool is a sublimation of feeling. Numbness is an inability to feel. Cool is a heightened form of emotional engagement — which is why it’s been the go-to secret sauce in most American art for the past 60 years. Numbness is a disengagement, a refusal, a tacit surrender to a 21st century that just won’t quit.

Which brings us to Shoreline Mafia, a young rap crew from Los Angeles that knows how to play numb in a cool way. The quartet — Ohgeesy, Fenix, Rob Vicious and Master Kato — headlined Songbyrd in Adams Morgan on Saturday night, rhyming about their ravenous pharmacological hunger and their meaningless sexual conquests. But instead of limping around the stage like a bunch of oversexed promethazine zombies, they each seemed completely present, totally alert. Sometimes, they even smiled.

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There’s plenty to smile about, for sure. After dropping a terrific debut album last November — a stylish bundle of songs that somehow felt nonchalant and meticulous at once — Shoreline Mafia has since become one of the most fascinating acts in a resurgent L.A. rap scene. And on Saturday night, the group’s chemistry was something you could practically reach out and touch. These guys love rapping, and even more than that, they love rapping with each other.

It mattered most during “Bands,” a sea breeze of a posse cut during which the foursome passed the microphone across the crowded stage like a relay baton. All together — from ­Ohgeesy’s pitter-pat trash talk, to Rob Vicious’s sturdy inflections, to Fenix’s sandpapery staccato, to Master Kato’s loose brags — the quartet somehow brought the urgency to a boil without anyone losing their cool.