出典:Wiktionary
know one's stuff (三人称単数 現在形 knows one's stuff, 現在分詞 knowing one's stuff, 過去形 knew one's stuff, 過去分詞 known one's stuff)
know one's stuff
a know-all
knowledge of something
知り合いになる
I know him
I am acquainted with him.
〈自分の能力を考えよ〉 You should know your limitations.