There are several words or phrases that can be used as synonyms for the phrase "knew one's stuff". Some of these include:
1. Proficient - skilled, expert, competent, adept
2. Well-versed - knowledgeable, experienced, familiar
3. Masterful - skilled, talented, accomplished
4. Accomplished - skilled, proficient, experienced
5. Adept - skilled, knowledgeable, expert
6. Competent - proficient, capable, knowledgeable
7. Skillful - proficient, talented, adept
All of these words imply that someone has a deep understanding or expertise in a particular subject or area. Whether it's in a personal or professional capacity, knowing one's stuff is a valuable asset that can be applied in myriad situations.
