It is a Spanish verbal tense, concretely "passive participle". It is like in English participle of "die" : dead. In Spanish, participle of "morir": muerto. Other examples in the same way for this word: "Muerto a balazos". (In this case, "muerto" is used like a synonym of "killed"). "Muerto el perro, se acabó la rabia". Another colloquial uses for "muerto": "Esto está muerto". ( When in a party there is a dull ambient, or there are not enough people). " Me he muerto de la risa". (When something is so funny than you can't stop laughing). " Estoy muerto". (When you're extremely tired). " Él es un muerto". (When somebody is a dull person, or when somebody is too heavy). "No me cargues el muerto". (When you say to somebody that you're not guilty of something). "Esto huele a muerto". (When something odours too bad).

¿Podrías poner más información sobre el texto donde lo encontraste? Lo que va antes y después de esta frase por favor. La frase podría tener sentido pero depende del contexto.

In Spanish we can have sentences without a verb! Let me explain. Maybe you have stumbled upon the common expression "¿Qué tal?". There's no verb. Some people say "Yes, there is, but it's omitted". Maybe. But we understand that sentence and there is no explicit verb. The sentence "Muerto mi padre cuando yo tenía nueve años" normally wouldn't be alone, and I bet it's followed by another sentence, separated by a comma. That's because in latin you could do especial constructions like that, called "Ablative absolute". In english it would be something like (and I know this is grammatically wrong, but maybe you will get the meaning by analogy): " *Done these things, the king and his wife left the castle" " *Dead my father when I was 15, I had to get a job" etc. There's a continuity meaning, but not by its own, it needs another sentence. It's, anyway, somewhat literary. You wouldn't hear somebody speaking like that on the bus.