Some substitutions for ground mustard are mustard seeds, commercially prepared mustard, wasabi powder or horseradish powder. If a a person only has mustard seeds instead of ground mustard, then he can grind up some of the seeds and use this as a replacement.
Another option is to use a commercially prepared mustard as a substitute. However, because there are different types of mustard (yellow, black or brown) with different heat and pungency levels, it is wise to use the same type specified in the recipe. Similarly, since wasabi and horseradish powders can impart a similar taste as mustard to a dish, they can be comparable substitutes.
Since ancient times, people used mustard as a condiment. The word mustard comes from the Latin words "mustum ardens," a spicy paste mixture that Romans made from ground mustard and unfermented grape juice called "must."