The word "mustard" comes from the way this condiment was made. Romans would take the seeds of the plant later called mustard, grind them to a mush, and add to this ground-up paste fresh wine, which was called (in Latin) "mustum." It has nothing to do with cursing or offensiveness unless you find the concept of must, the juice of fresh fruit (especially grapes, hence the connection to wine), in some way offensive. Are you think of the similarity of "mustard" to "basta rd? The two are not etymologically connected.