The ultimate origin of this word is probably Egypt. The world's harvest of black and brown mustard seeds is only to a small part used for the production of mustard paste, since the pungency of black mustard paste is not as stable as the pungency of white mustard , but diminishes after some time. To stabilize the pungency, these pungent mustards usually contain acidic components. See white mustard for more information on mustard pastes. See also sichuan pepper for a comparative review on pungent spices.
Black mustard is more important as a spice and oil plant, especially in India.
Owing to paternistic laws, in most Western countries mustard oil is not a legal food additive and its sale is prohibited. You needn't take this remark serious if you know about the heating procedure. See sesame for a general discussion on plant oils. Their pungency is completely destroyed by cooking, and therefore the ground seeds should be added as late as possible.