拿小拳拳锤你胸口
Ná xiǎo quánquán chuí nǐ xiōngkǒu
This was popularised after a girl used this in a wechat conversation with her boyfriend.
If we break down the phrase:
拿小拳拳: directly translating this means 'using my small fist fist'. Grammatically, this is wrong but instead, this uses a 'cute' way of speaking. The fact that the word 'small' is added alongside the repeat of the word 'fist' is to emphasis on the cuteness of the person speaking this.
锤: this means to hit, usually used with objects like hammers
你胸口: this means your chest
Therefore, this means "to use my fists to hit your chest".