Robert "Rabbie" MacNab is the son of Ronald and Mary MacNab. As a child, he works at the manor house of Lallybroch as a stable boy. As an adult, he lives in London and runs a brothel with his wife Agnes.
Rabbie grew up with a doting grandmother and scads of aunts, uncles and cousins. Still, he knew violence in his home life, as his father was a drunk who often took a rough hand to the boy.
After discovering Rabbie's home situation, Jamie Fraser arranged for Rabbie to work at Lallybroch as a stable boy. Rabbie's father, Ronald MacNab, was angered by Jamie's actions and eventually betrayed Jamie to the Watch, leading to Jamie's capture by redcoats and subsequent imprisonment at Wentworth Prison. As justice, the tenants of Lallybroch executed Ronald MacNab by setting fire to his house, and Rabbie and his mother came to live in the manor with the Murray family, working as stable boy and kitchenmaid respectively.
Claire tends to Rabbie after he experiences an epileptic seizure which, according to his mother, had afflicted the boy for about two years. Claire surmises that the fits are a result of damage inflicted by his late father, who took a violent hand to his son, but Mary MacNab remains convinced that her son is somehow possessed by demons. Knowing that there is no available treatment for the boy, other than to protect his head while a seizure happens, Claire finds inspiration to allay at least the mother's worry: Claire instructs her to sew a small charm stone into Rabbie's pocket, which will protect him from "the devils" that afflict him during a fit, and ensure that he comes out of it all right.
In summer 1757, Rab lives in London and works as a chairman. Nessie tells Lord John Grey that Joseph Trevelyan does not go with whores at the brothel she works at but instead goes into Magda's office, which then a "woman" in a green dress like Mags is taken by chair. Nessie brings Rab up saying that he is a chairman and may be able to say where the lady in green goes. Grey pays her extra to ensure that Rab has the coin to see her very soon.
Once Rab has spoken with Nessie he is directed to Grey's home on Jemmy street. Upon Rab’s arrival Grey sets aside his mail to learn what the chairman could offer. As Grey and Rab negotiated the cost of his information Rab offers half his price to Nessie and declares he intends to marry her - once she buys the freedom from her contact. Rab tells Grey that the lady in green goes to Lavender House on Barbican Street. Rab, of course, having no idea the lady in green wasn’t a lady or the true nature of Lavender House, but Grey knew the house and that this investigation was going to take him to this place from his past.
Rab and Nessie are married and together they run a brothel. In early 1758, they save Lord John Grey, Nessie's acquaintance, when Grey is attacked on the street.
As an adult Rabbie is tall and solid in build with a stupid, sullen face. His hands are thick as horn, broad and bluntly powerful, with yellow callused palms. His hands are otherwise gnarled, frozen in a curve from his occupation as a chairman.
- Robert is from the Germanic name Hrodebert meaning "bright fame", derived from the Germanic elements hrod "fame" and beraht "bright".
- MacNab is derived from the Scottish Gaelic Mac An Aba, which means "child of the abbot".
- Main article: Outlander (TV series)
On the show, Rabbie's situation is much the same as in the books. After Claire Fraser witnesses Ronald MacNab's rough treatment of the boy at the Quarter Day celebration, she confronts the elder MacNab and offers to watch Rabbie for a while. She takes the boy inside where she and Jenny Murray discover bruises on his back. Jenny takes Rabbie to get cleaned up and something to eat.
Later that day Jamie Fraser confronts Ronald MacNab and fights him. The next day, Jenny informs Jamie that MacNab had said that if Jamie thinks he can do a better job, then Jamie can pay to feed him, and leaves Rabbie with the Frasers, where he begins work as a stable boy.