The term "lamb to the slaughter" is usually used figuratively rather than literally. As a figure of speech, it means an innocent person who is being set up as a sacrifice or dupe without realizing what is going on.

At first, it seems as if the innocent, sixth-month pregnant Mary—who looks like the Virgin Mary with her translucent skin and big eyes—is the lamb to the slaughter: we find out that she is being sacrificed to her husband's desire for a divorce, presumably so he can marry another woman. She seems to have done everything to be a good wife. She seems to be an innocent victim of her husband's selfishness.

Situational irony is when the opposite happens from what is expected in a story. Verbal irony occurs when words mean the opposite of what a reader expects. In the phrase "lamb to the slaughter," Dahl uses both situational and verbal irony. Mary, unexpectedly, is not the lamb to the slaughter: the situation works out in the opposite way. Sweet Mary slaughters her husband. And in a use of verbal irony, she literally uses a lamb (leg of lamb) to slaughter him. The phrase "lamb to the slaughter" is literal, not figurative, because she whacks her husband on the head with a frozen leg of lamb and kills him.

The irony of the title lies in its ambivalent implications; through his fantastic story, Roald Dahl introduces an unlikely twist to the way this famous phrase is interpreted. He chooses to focus on a more direct interpretation rather than emphasize the conventional metaphor we are used to.

The phrase "lamb to the slaughter" is typically an allusion to Christ being led as an innocent to his undeserved death. In this story, the narrator chooses to use the phrase to characterize an actual leg of lamb as a weapon. The leg of lamb is literally the instrument of slaughter.

In the story, Mary Maloney is a faithful and loving wife; she is a woman who enjoys waiting on her husband. One evening, her husband comes home visibly agitated, only to confess that he has decided to leave her. The news shocks Mary to the point that she finds it difficult to function. At six months pregnant, Mary is in an unenviable position.

After her husband's refusal to eat dinner and his injunction that she not make a fuss, Mary whacks him on the back of his head with a frozen leg of lamb.

In R. Dahl's short story "Lamb to the Slaughter," what is ironic?

In Roald Dahl's "Lamb to the Slaughter," the greatest irony in the story is how Mary Maloney literally gets away with murder with the help of the police—and they never realize it.

Generally, irony is the difference between what one expects to happen and what really happens.

Irony makes use of hyperbole, sarcasm, satire, and understatement.

Mary Maloney is pregnant and a devoted wife. One evening her husband, a policeman, comes home from work and tells his wife that he is leaving her. Much more concerned with himself (and how this will affect him at work), he is unaware that his wife is so stunned.

Almost in a daze, Mary goes to the basement to get something from the freezer for dinner. Even as he hears the sounds of preparation for a meal, Mary's husband tells his wife he doesn't want anything to eat—but he gets much more than food: for as he gazes out the window, Mary approaches him from behind and bashes him in the head with a frozen leg of lamb, killing him. There is some irony here, too: he doesn't want food; she doesn't give him food—but uses the "food" to kill him.

As he lies on the floor, Mary worries about what will happen to her if she is caught. Leaving his body where it has fallen, she puts the lamb in the oven to cook. Then she runs to grocer as if she needs more things to make an unexpected dinner for her husband (since they usually go out for dinner on Thursday nights). This provides her with an alibi. Returning home, ostensibly to find her husband murdered, she calls the police. They arrive and start looking for clues. Unbeknownst to them, the murder weapon is not only being cooked in the oven, but it will eventually be their dinner. There are several forms of irony at the story's end. First, one officer imagines that the weapon might be right there in front of them. (This is dramatic irony: Mary understands the irony, but the policemen do not.)

One of them belched.

“Personally, I think it’s right here on the premises.”

“Probably right under our very noses.  What you think, Jack?”

The other example of irony (called "situational irony") is that the men have disposed of the murder weapon themselves. This is not what one would expect the police to do: and they do it with no knowledge that they have done so.

The woman stayed where she was, listening to them speaking among themselves, their voices thick and sloppy because their mouths were full of meat.

There is irony, too, in that the men not only eat, but that they finish the meat: as a favor to Mary, for she tells them she couldn't eat a thing. Dahl's story abounds with instances of irony, which make it darkly entertaining.