adjective [ADJECTIVE noun] If you describe an action as dastardly, you mean it is wicked and intended to hurt someone. [...] [old-fashioned]

What is a dastardly act?

cowardly; meanly base; sneaking: a dastardly act.

Is dastardly a bad word?

Someone who's dastardly is a scoundrel or a cowardly villain. A superhero might try to protect a city from a dastardly criminal mastermind. Use the adjective dastardly to describe a person or action that's underhanded and mean.