没心没肺姑娘刀枪不入
[méi xīn méi fèi gū niáng dāo qiāng bù rù]
This nickname describes a girl who is so carefree and unbothered by external factors that she seems invulnerable or unaffected. It implies someone who doesn't worry too much, perhaps lives without many cares, hence 'heartless and fearless' (metaphorically speaking) leading to being invincible against troubles or pain.