Casaba is the imperfect subjunctive form of the verb casar, meaning "to marry". While casar is common, this specific conjugation is less frequent in everyday conversation and adds a layer of hypothetical desire.
In the song, Christian Nodal sings, "Contigo yo sí me casaba" (With you, I would indeed marry). Despite acknowledging the toxic nature of their relationship, he expresses a deep, almost unconditional commitment, making this word a powerful and somewhat surprising declaration of love within the context of the lyrics.
"Amor Tóxico" tracks a relationship that never settles into one mood. "Un día me amas" ["One day you love me"] gives way a line later to "Luego me odias y no quieres ver mi cara" ["Then you hate me and don't want to see my face"] — jealousy over a photo, a whole imagined scandal from one message, a breakup undone by morning: "Un día quieres terminar y al otro regresar" ["One day you want to end it, and the next you're back"]. Nodal doesn't dress the pattern up; he names it right in the hook, "Mi amor tóxico del alma" ["My toxic love, to my very soul"], and keeps choosing it anyway, gossip be damned: "Y lo que digan que nos valga" ["And whatever they say, we don't care"].