In January 1817, Jane Austen wrote to her niece Caroline Austen:
'Your Anne is dreadful - . But nothing offends me so much as the absurdity of not being able to pronounce the word Shift. I could forgive her any follies in English, rather than the Mock Modesty of that french word...’
She thinks it’s a little ridiculous that this Anne, a character in Caroline’s own writing, insists on using the French word chemise. Whether it’s a person or a fictional character, Austen is against this affected sort of pretentiousness. Presumably, she sees no reason to be squeamish about the word shift.
But what is a shift, and why would someone call it a chemise?
A shift was just a woman’s undergarment. Men wore shirts and women wore shifts. A chemise d'homme was a shirt and chemise de femme was a shift. Think of a shift as the Regency equivalent of an undershirt.
Call it what you want, but why did a regency lady need to wear a shift? It’s a garment worn next to the skin to protect the wearer from her stays pinching her, but more importantly it’s meant to protect the outer garments from sweat and oils. A shift was intended to be worn and washed daily; a gown was not. Although sleeve length varied, shifts remained the same throughout this period: loose fitting, to about the knee, with either a fixed or drawstring neckline and sleeve gussets in the armpits.
Over the shift/chemise, they laced up stays and then put on a petticoat, which was a long slip that was held up with shoulder straps and tied at the waist. Other than stockings, that was the entirety of what regency ladies wore under their gowns. Shift, stays, petticoat, stockings.
This is from the Metropolitan Museum of Art––and they call it a chemise even though it's dated from 1780-1800.
Was there a difference between a woman’s shift and a chemise? They both referred to the same linen undergarment. The word “shift” was used through the early nineteenth century and transitioned to the French word. As the word “chemise” became the standard in English, the patterns became more elaborate and the chemise was more often made of cotton. But in this time, was there any difference? No.
In your regency-set reading or writing, you could use the word shift or chemise, but Austen would probably roll her eyes at your affected delicacy if you use chemise instead of the plain English word shift.