What is undercriminalization?

Answer 1

Under-criminalization is any law that's intent is to make illegal but falls short in making something completely illegal. Or omits a specific type of wording to make any certain type of action illegal.

e.g. Pants and Shorts are found to cause cancer so the FDA bans it. However in the law they forgot to mention that denim is the cause and so silk pants are banned, however, denim jeans and shorts can still be worn, though this was not intended.