What is the meaning of Smirch?

Dirt, or a stain.

A stain on somebody's reputation.

To dirty; to make dirty.

To harm the reputation of; to smear or slander.

A chirp of radiation power from an astronomical body that has a smeared appearance on its plot in the time-frequency plane (usually associated with massive bodies orbiting supermassive black holes)

Source: wiktionary.org