ser un plomo
What does this idiom mean?
3 Answers
un plomo is someone who helps musicians on stage, tune their instruments, check sound and get everything ready before, during and after the show. A rodie. Here´s a nice video on the subject. .
But also, ser un plomo (also, plomazo, moplo) = to be a pain the neck, to be a pain in the a....**** (you know), to be a pain in the backside, to be a pain in the proverbials.
If you´ve seen this movie, John Candy personifies the master of plomos

Ser un auténtico pesado.
In the sense of "pest" o "annoying".
To be a drag/bore/party pooper/dead weight/etc.