When it comes to synonyms for the phrase "knew stuff," there are plenty of options to choose from. Some possible synonyms include "knowledge," "expertise," "information," "understanding," "awareness," "familiarity," and "insight." These words all refer to a person's possession of facts, ideas, or skills in a particular subject area. Other synonyms might include "know-how," "wisdom," "savvy," "knack," "intelligence," or "cognition." Regardless of the specific term used, each synonym implies a degree of familiarity and proficiency in a given field, suggesting that an individual has the ability to use their knowledge effectively and make informed decisions.