"Narrow back" is an affectionate Irish expression describing someone — typically a woman — with a slender, graceful figure, especially a tapered waist and back.
Unlike "skinny" (potentially negative) or "thin" (clinical), "narrow back" carries warmth and admiration — implying elegance, youth, or attractiveness. It's not derogatory.
Traditionally applied to women, but modern usage extends to anyone with a slight build. The phrase reflects Irish English's rich somatic vocabulary — precise, poetic body descriptions embedded in everyday speech.