tomar a alguien por el pito un sereno 9
Ignore him, ignore him, pay no attention to him or his actions or opinions, not take him into account at all. The locution derives from the abuse of the whistle by the watchmen in the face of any nonsense at night. The neighbors ended up ignoring them when they blew their whistle. Some did pay attention when they said, "Twelve and serene," or "Five and snowing," when they were warm in bed.