In Shakespeare's Macbeth, when Lady Macbeth tells Macbeth that he is infirm of purpose in Act 2.2, she means that he is not solid or concrete, that he is not firm, in his determination to go through with what he wants and needs to do. This is an insult. She is berating him, because he's afraid to take the bloody daggers back to Duncan's chambers. Of course, Macbeth was also an idiot for bringing the murder weapons with him in the first place.
She continues to berate Macbeth by making fun of him in other ways for not being willing to go back to Duncan's chamber to return the daggers. She ridicules him by saying that a dead body is just like a picture: harmless, of course.
And, she says, only a child is afraid of a painted picture, even if it is of a devil. That's the eye of a child and painted devil part.
Not only does Macbeth foolishly bring the daggers back with him to his wife, but he is afraid to take them back once she discovers them. And she berates and ridicules him for it.