The -ard is basically “fucking” + nominaliser (if necessary):

• wizard - fucking wise one
• drunkard - fucking drunk one
• coward - fucking tail (the initial part is from Old French “coue” tail, itself from Latin “cauda” tail. Who shows the back in a fight? Someone running away!)

The “nominaliser” part is an artefact of the borrowing, the suffix is from French. Romance languages often use adjectives as if they were nouns, but that doesn’t quite roll in English. In turn French borrowed from Frankish.

The etymology of “mustard” is disputed. The first part is likely from Latin “mustum” must; it used to be prepared with young wine. The -ard is typically explained as ardens (fiery, hot). So basically “mustum ardens” hot must. …Capsicum peppers are from the Americas, black pepper and long pepper were expensive, European mediaevalards didn’t really have a lot of spicy flavours to work with, so… I guess mustard was spicy for them?