Don Ramón explica el veneno con una calavera

What does “irse al otro barrio” mean in Spanish?

A skull on a bottle means poison, and if you drink it you’ll head al otro barrio — “to the other neighbourhood,” a jokey Spanish euphemism for dying. The afterlife reframed as just another part of town keeps the topic light. Flip it to the active mandar a alguien al otro barrio (“send someone to the other neighbourhood”) and it means to kill them.

Note the article: el otro barrio with the definite article is the realm of the dead, while otro barrio without it is simply some other district across town. The expression belongs to a family of dark-comic Spanish death idioms (estirar la pata, criar malvas) used precisely to defuse the fear by joking about it.