Based on the search results, there is no clear definition or explanation of what a "bad coupon code" is. The search results include articles and guides about promo codes and gift codes for various games and products, as well as a Reddit post about using random codes when online shopping to potentially get a discount. However, none of these sources provide a definition or explanation of what a "bad coupon code" is. It is possible that the term is used colloquially or within a specific context that is not covered by the search results.