Casi algo literally translates to "almost something". It's a popular modern term to describe a relationship that is more than a friendship but never becomes official—a romantic situationship that ends before it truly begins.
In the song, Xavi uses this phrase to define a failed romance. He refers to the person as his casi algo (almost something), casi todo (almost everything), and casi siempre (almost always) to express the bittersweet feeling of what could have been, but never was.
Ever been in a situationship that dragged on way too long? In "Tu Casi Algo", Mexican singer Xavi and Fabio Capri lay down the law for an ex who keeps calling. The title literally means "your almost-something" (the Spanish phrase for an almost-relationship), and the singer is totally done pretending this was true love. Right from the start, he tells her to stop messing with his life ("Deja de estar jodiéndome la vida más") and even bets her cold hard cash ("te paso una feria") that she will never find another guy better than him.
The track is packed with colorful Mexican street slang. He tells her "túmbate el rollo" (drop the act / quit your nonsense) because he already knows all her sneaky tricks and smooth talk ("labia"). Instead of dragging out the drama, he asks for a quick favor ("hazme un paro"): delete all the photos right away ("en corto"). He has already erased her from his life, leaving her to deal with the loss while he signs off as her former "casi todo" (almost-everything) and "casi siempre" (almost-always).