A person who lacks intelligence.Can be at very age.It means the person doesnt have a lot of knowledge in different situations but mostly does actions which normal people consider to be uncommon and not responsible.Also they may find it difficult to obey a certain rule.
"I told her 3 times not to eat others leftovers but she still doesnt understands it,she is so stupid."
"She really got into relationship with my ex...gosh she is stupid!