妞 is very informal and can sound flirtatious or objectifying depending on context. It's common in northern dialects, especially Beijing. Use with caution.
Woman radical on the left - anchors 妞 in the family of female-kin and female-quality characters alongside 妹 (younger sister), 姐 (elder sister), 妈 (mother), 好 (good), 婶 (auntie). The radical signals 'female being'; the phonetic supplies the colloquial 'girl/lass' nuance.
Right 丑 supplies the sound - chǒu drifted to niū. Pure phonetic loan; the 'ugly' meaning of 丑 contributes nothing - 妞's connotation is affectionate, not pejorative. 丑 also names the 2nd earthly branch (Year of the Ox).
