待我长发及腰定装鬼吓死你
[dài wŏ zhăng fā jí yāo dìng zhuāng guĭ hè sĭ nĭ]
In this humorous context, it refers to 'When my hair grows down to my waist, I will definitely dress up as a ghost to scare you.' It combines humor and playful menace into the waiting time of long hair growing.