While reading Villette, a 19th-century novel by Charlotte Brontë, I came across the word guard used in a different way than I had known hitherto:
I was up the next morning an hour before daybreak, and finished my guard, kneeling on the dormitory floor...
From the above sentence I infer that the word carries a religious connotation—a morning prayer, a sermon or perhaps a rite.
The closest definition was 'posture of readiness or defence'; however, it doesn't coincide with the overall meaning of the sentence as I, perhaps wrongly, understand it.
The woman speaking is a Protestant, a detail that may or may not prove to be useful.