This is a MESSED UP story, so if you have any issues with serial killers or violence towards children, this is your trigger warning. I'd heard about this story numerous times (it's one of those sensation things that's always floating around academic circles), but I found this particular excerpt on Futility Closet's blog here.
"On Aug. 24, 1867, solicitor’s clerk Frederick Baker took a tea break and strolled into the meadows near the hop fields of Alton in Hampshire. There he found three little girls [the two Adams sisters and their friend]. He played with them, running races and picking blackberries, then dismissed two of them with three halfpence. They watched him carry 9-year-old [I think she was actually 8, but that's a minor quibble] Fanny Adams up the hollow, telling her, 'Come with me, and I will give you twopence more.'"
From what I understand from other sources, Fanny did not want to go with him, so he carried her away, though not in a manner violent or threatening enough to alarm her friend and sister. The other two girls eventually returned home in the evening and only told an adult what had happened when they were asked directly by a neighbor where Fanny was.
The neighbor alerted their mother, and the two of them went off in search of Fanny. On their walk, they encountered Baker on his way back. The identified him from the girls' description, and he happily admitted to giving the children money for candy, but claimed he had no idea where Fanny was. As he was a respectable looking man (being a solicitor's clerk), they let him pass. Mrs. Adams was wife to a bricklayer, so there was most definitely a class divide, however slight. Even if she had called for the authorities on that point, it is highly unlikely that they would have held Baker, especially since there was no evidence that anything bad had befallen Fanny.
"Searchers found Fanny’s head on a hop pole. Both eyes had been gouged out and one ear torn off. Her arms were found in two locations, one hand still holding two halfpennies. Her heart had been scooped out of the upper torso, one foot was found in a field of clover, and her legs were assumed to have been taken by River Wey. There was no evidence of sexual assault because her lower torso was never found."
The police, at the head of an angry mob, went immediately to Baker's law office. They found blood on his clothing that he couldn't explain, and two small blood-stained knives. He maintained that he was innocent, stating that his knives were too small to have killed Fanny in that manner.
However, before the police arrived, a coworker testified that Baker had indicated that he was going to leave town. The coworker told him he might have difficulty finding another job as a clerk, to which Baker replied, "I could go as a butcher."
The most damning piece of evidence was found in Baker's own diary, written in his hand. His diary entry on the day of the murder read: 'Killed a young girl. It was fine and hot.'
He was obviously arrested, and the police had difficulty protecting him from the waiting mob. He went to trial a few months after and his legal team tried every trick in the book. They argued, again, that his knives were too small to mutilate Fanny in that way, but even if he DID do it, there was a lot of hereditary insanity in the family, so he was clearly crazy and shouldn't be held responsible for his actions. They even brought up epilepsy as a possible cause.
When that didn't appear to be working, they argued that the grammatical structure of his diary entry was incorrect to be admitted as a confession, and tried to get that thrown out of court.
The judge urged the jury to find him not guilty by reason of insanity, but after only 15 minutes of deliberating, the jury ignored the judge's request and found Baker guilty. He was hanged on Christmas Eve.
The phrase "Fanny Adams" took a very darkly humorous turn two years later, in 1869. New rations of tinned mutton were distributed to sailors. Unimpressed by both the quality and the quantity, the sailors took to joking that the mutton might really be the unfound remains of Fanny Adams (especially since the parts of her that were never recovered were assumed to have been swept away by the river and into the ocean).
"Fanny Adams" became a slang term for mutton, and then for poor-quality meat or food, and then for something of no quality whatsoever.