姑娘你内衣带子露了
[gū niáng nĭ nèi yī dài zi lù le]
This translates to 'Young Lady, Your Bra Strap Is Showing'. While on face value it's simply stating an observation that might be shared among friends in private, it could also be considered intrusive or embarrassing depending on the context and relationship. Whoever chose this nick name might have some story related to fashion sense or awkward situation involving attire.