Furiosa means "furious" or "enraged." It's a strong, evocative adjective that perfectly captures the intense emotion in the song.
Becky G uses this word to describe her reaction when someone tries to take her love interest. She sings, "Y si alguien me la quita, me pongo, pongo furiosa" (And if someone takes her from me, I get, get furious), highlighting her possessive and passionate nature in a very direct and memorable way.
"2AM" is an inventory. Becky G goes down the list of what belongs to her — "Míos tus labios, mía tu piel" ("Your lips are mine, your skin is mine"), "Míos tus ojos, mío tu ser" ("Your eyes are mine, your being is mine") — and keeps going past the body into the ordinary business of being alive: "Lo que respiras, lo que bebes" ("What you breathe, what you drink"). Then the list reaches the one item she cannot claim: "Te veo con otra y ay-ay-ay" ("I see you with someone else and ay-ay-ay").
She hears how that sounds and gets ahead of it. "Y no es que sea celosa" ("And it's not that I'm jealous") arrives two lines before "Y si alguien me la quita / Me pongo, pongo furiosa" ("And if someone takes her from me / I get, I get furious"). What she asks for instead of an apology is a deletion: "Que los besos que te dieron / Lo has borrado, no existieron / Los te amos no valieron / Antes de mí" ("That the kisses they gave you / You've erased them, they never existed / The I-love-yous were worth nothing / Before me"). The chorus then returns four more times, which is the point — the counting never finishes.