大叔(dà shū) - uncle, a friendly way of addressing a middle-aged man in Mandarin

大叔 (dà shū) is what you call a man roughly of your father's generation when he is not actually a relative: the bloke at the fruit stall, a stranger you are asking directions from. Mandarin prefers a family term to a neutral one here, and this is the standard choice for a man in his forties or fifties.

Its counterpart for a woman of the same age is 阿姨 (ā yí), and for a younger man 大哥 (dà gē) or 哥 (gē). Pick by apparent age: calling a thirty-year-old 大叔 (dà shū) will land as an insult, and calling a sixty-year-old 大哥 (dà gē) sounds like flattery.

Do not confuse it with 叔叔 (shū shu), which is the genuine kinship term for your father's younger brother and also what small children call any adult man. 大叔 (dà shū) is the adult-to-adult version, informal but perfectly polite.