This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
Brassica is Latin for cabbage, and arvensis means belonging to arable land.
It is called Charlock; Brassies (from the Latin which was used in old leases, in which were conditions as to its being kept under); Corn Kale, so called when hawked as a salad, before flowering, in Dublin. It has been grown as a salad, and mixed with Black Mustard as mustard. It contains an oil, and the seeds are hot and acrid.
Essential Specific Characters: 34. Brassica arvensis, O. Kuntze. - Stem thick, hirsute, purple at the joints, radical leaves petiolate, sublyrate, upper sessile, dentate, flowers yellow, large, pods knotted, subcylindrical, many-angled, with conical beak.