It's important to note that this phrase is considered offensive and disrespectful and should be avoided. It can be hurtful and is often used to make fun of or belittle someone.
Instead of using this phrase, consider using more respectful and accurate language to describe someone's understanding or cognitive abilities. For example, you could say:
* "They're a bit slow on the uptake."
* "They're struggling to grasp the concept."
* "They haven't quite understood the instructions yet."
Remember, everyone learns and understands things at different paces. It's important to be patient and supportive, rather than using hurtful language.