As well as being incredibly strong, Molly's wrists are covered with scars. Once upon a time, Molly was a serial criminal who was saved from the gallows by the expert legal defense of her current employer, Mr. Jaggers. While she was on trial for murder, Jaggers instructed Molly to cover up her wrists so that the jury wouldn't think she was strong enough to commit such a crime. The ruse worked, and as we have seen, Molly was acquitted.
The presence of scars on Molly's wrists indicates the wearing of manacles, a common practice even among petty criminals in those days, let alone suspected murderers. The escaped convict Abel Magwitch is also shackled when he first meets Pip at the graveyard. But in his case, he's shackled by leg-irons which are designed to keep him in the hold aboard a prison-ship.