"Dull-headed" is a term used to describe someone who is slow to grasp new ideas or has a lack of mental acuity. A few synonyms for this word could include "dense", "thick", "dim-witted", "stupid", or "slow-witted". Other options could be "unintelligent", "unperceptive", or "unobservant". However, it's important to remember that using derogatory terms to describe someone's intellectual abilities can be hurtful and counterproductive. Instead, we should all strive to treat each other with kindness and empathy, recognizing that everyone has their own unique strengths and weaknesses.