What is the origin of the idiom "Mrs. Grundy" and when to use it?

The idiom "Mrs. Grundy" originated from a character named Mrs. Grundy in the play "Speed the Plough" by Thomas Morton, which premiered in 1798. It is used to express the fear or concern of conforming to societal expectations or the pressure to maintain a respectable image. The idiom is employed when discussing the influence of public opinion or the fear of social stigma in shaping one's actions or decisions.