The two most significant symbols are 'lamb' and 'supper'. Both of these are used in a profoundly ironic context on many different levels.
A lamb is deemed an innocent, harmless animal. It is feeble and non-threatening, and, dare we say, quite stupid. Mary, in this context, may be represented as the lamb for, throughout the story she presents a docile, servile attitude. She does her best to please her husband, and, like the innocent and unsuspecting lamb, is punished for her love, kindness and unquestioning duty to him. He unscrupulously discards her like a rotten rag, even though she had obviously been utterly at his service throughout their marriage - and she is due to further bless him with a child. He arrogantly informs her about his decision to leave, expecting her not to do anything about it. The irony is that the well-serving and docile lamb does decide to do something and, at a whim, she murders him. She then, unlike the proverbial lamb, intelligently masks her guilt by creating an alibi and cleverly disposes of the murder weapon.
Patrick Maloney, in this context, could also to some degree be seen as symbolic of a lamb - not in the same sense as described above, but as the unsuspecting victim of a killing. Just as a lamb would be unsuspecting of its fate when it is led to slaughter, is Patrick Maloney caught completely unawares. He could hardly have expected any form of retribution from his wife, least of all murder!
In a literal context, the leg of lamb was used to 'slaughter' Patrick Maloney. In an ironic twist, the frozen leg of lamb, which, to add further irony, is the product of a lamb which had also been slaughtered, has become a weapon. In this sense, then, the lamb is truly taken to slaughter - in this case to slaughter the victim.
'Supper' denotes a period of satisfaction - the last big meal of the day, followed by relaxation and rest. In the story, Mary's insistence that her husband have supper is symbolic of her desire to serve and please him. It is an indication of her profoundly servile attitude to him - a fact that Patrick Maloney seemingly does not appreciate. It is this 'supper' which ultimately leads to his death, for Mary had initially take the leg of lamb out of the freezer to cook for him, but then uses it as a murder weapon.
Furthermore, the supper is symbolic of "The Last Supper" depicted in Christian literature, when Christ enjoyed his last meal with the disciples before his arrest, incarceration, trial and final crucifixion. Ironically though, Patrick does not enjoy this supper since he is killed before it is prepared.
Mary Maloney uses the supper as an excuse to create an alibi. In this regard, the supper becomes a symbol of her deception. She cleverly encourages the investigating policemen to enjoy the meal and finish off the lamb, thus getting rid of the evidence completely! It is ironic that one of the men should remark: "It's probably right under our noses" whilst enjoying the most incriminating proof.
The first symbol could be the obvious use of the word "lamb". Look at the fact that the title suggests that an animal will be killed. What do we know about lambs? They're weak, they're feeble, they're unimportant, and lastly, they're followers. So, with all of this background knowledge about lambs, you can tell that Ronald Dahl wanted to create a pun, since Mary Maloney seems to be the weak, unimportant, feeble character throughout the story. Mary is the lamb; she dutifully follows her husband--doing all she can to please and appease him as a wife. What was her repayment? A divorce!
The idiom "lamb to the slaughter" suggests something innocent that is willingly, in fact, blindly following something that will ultimately lead to its demise. Mary (also an allusion to the children's sing-a-long "Mary had a little lamb") was the lamb, but just like in classic fable stories, she becomes the strong one as she overpowers her husband, killing him with a leg of lamb.
The other symbol would be the religious implication of the word "lamb". In the bible, a lamb is sacrificed (Jesus Christ) for the sins of the world. In the story, the lamb is not sacrificed for sins, it is rather, being sacrificed as atonement for wrong doing. Patrick Maloney had done his wife wrong by falling out of love with her after years of service and devotion. Therefore, in the spirit of atonement, he was to be sacrificed.