If something isn’t sharp, it’s dull. This can apply to pencils and people — if you’re sharp, you’re a smarty-pants, but if you’re dull, you’re a dullard. It even sounds kind of dumb, dullard.
Dullard is an old-fashioned word for a dumb person. If you have to explain to someone how to sit in a chair, you’re probably talking to a dullard (or a toddler). It's rude to call someone a dullard, but we all feel like dullards sometimes, especially when we make mistakes or can't understand something.