The antonyms for the phrase "knows stuff" can vary depending on the context. Some antonyms could be ignorant, uninformed, clueless, or uneducated. These terms all imply a lack of knowledge, understanding, or expertise in a particular subject or field. On the other hand, synonyms for "knows stuff" may include knowledgeable, savvy, skilled, or proficient. These terms suggest that someone is well-informed, experienced, or highly capable in a particular area. It is important to note that antonyms for "knows stuff" shouldn't be taken as an insult, as we all have different strengths and weaknesses when it comes to knowledge and expertise.