I think the explanation is that Chinese is not only written but spoken, and a lot of characters, while used in idiomatic daily speech, don't really have a formal written counterpart. If you ask locals to actually write this character out, most likely they'll write a commonly used character with the same pronunciation. However, language experts who wrote the dictionary insist on assigning a formal written form to each character out there, and they recognize the fact that this character has a totally separate meaning than the commonly used surrogate character, thus they likely invented this new character just to represent this idiomatic sound. This is a common phenomenon if you browse through modern Chinese dictionaries.
板着(脸)
means…