A mustard bath is a traditional English therapeutic remedy for tired, stressed muscles, colds, fevers and seizures. The mustard was thought to draw out toxins and warm the muscles, blood and body. It was a standard medical practice up until the first part of the twentieth century and continues to be used in alternative medicine.
The ancient Greeks and Romans, Europeans, and the Ayurvedic tradition of India have all used mustard in this way.
Around half a cup of ground-up mustard seeds are placed in a very hot bath, and the patient relaxes in this for about ten minutes.
References
Mustard bath Wikipedia(Text) CC BY-SA