When your friend said, "you are dumber than dogshit," they were using a harsh and hurtful expression to convey that they think you are extremely unintelligent or foolish. Comparing someone to "dogshit" is a crude and offensive way of suggesting that they believe you lack intelligence or common sense to a significant degree.
It's important to note that such comments are disrespectful and can be emotionally damaging. Friends should be supportive and encouraging, not insulting or demeaning. If you feel hurt or offended by their remark, it may be worth having a conversation with them about how their words affect you and expressing your desire for more respectful communication in your friendship.