The title might be called a "triple entendre." First, a frozen leg of lamb is used as the murder weapon--a lamb is used to "slaughter" Mary's husband. Second, Patrick Maloney is killed like a lamb led to the slaughter. The title is derived from the Bible.
He was oppressed, and he was afflicted, yet he opened not his mouth: he is brought as a lamb to the slaughter, and as a sheep before her shearers is dumb, so he openeth not his mouth.
A lamb going to be slaughtered would go quietly and trustingly, since it would be too young and innocent to know where it was being taken. Patrick is similar to a lamb going to the slaughter because he puts up no resistance and has no idea what is going to happen to him. Who would ever imagine the meek and mild Mary Maloney doing such a thing as murdering her husband with a frozen leg of lamb--or anything else?
Thirdly, Mary herself is like a lamb. She is a very quiet, docile, devoted housewife who loves her husband more than anything else in the world. Yet she goes to the slaughter as the slaughterer. She unveils an amazing side of her character which continues to sustain her while she is scheming to commit the perfect crime. Only we the readers know that she is capable of committing a brutal murder, and so we know that she is capable of covering it up. She shows that she has been a cop's wife for a long time when:
"I think it's a shame," she said, "that when a policeman gets to be as senior as you, they keep him walking about on his feet all day long."
This means that she knows a lot about the way police investigators think and act. She continues to behave like a simple lamb, but we know she is no lamb anymore.