It is primarily an old-fashioned insult for a "stupid" or "dumb" person.
There is a notable distinction in modern usage, particularly in groups like the "Dull Men’s Club," where members embrace being "dull" (having mundane or unexciting hobbies) but object to being called "dullards," which they interpret strictly as an insult to one's intellect.
is a noun used to describe a person who is perceived as slow-witted, unintelligent, or unimaginative. While it shares a root with the adjective "dull" (meaning boring or lacking excitement), it specifically implies a lack of mental sharpness or intelligence. Key Meanings and Nuances
If you are looking for alternative words, they range from mild to strong insults:
