扎心了老铁 or 扎心了! 老铁
(Zhā xīnle lǎo tiě)
First, let me explain the individual terms within this phrase.
老铁 (lao tie): It is a slang used usually by those from the Northern parts of China. It is used to refer to someone you know and is close to, and it signifies a sense of brotherhood. It is similar to the word 'Brother/Bro' in English.
However, for those from the previous generation in the past, it used to signify those illegitimate relationships such as mistresses but the meaning was more implicit.
扎心了 (zha xin le): 心 refers to the heart while 扎 is an action term referring to clutching/tying. Therefore, this phrase means that your heart is in much pain, a sensation of someone clutching your heart.
An example of the phrase in action:
"单身狗还要陪好朋友去选送女友的情人节礼物，真的，扎心了老铁。"
(Dānshēn gǒu hái yào péi hǎo péngyǒu qù xuǎnsòng nǚyǒu de qíngrén jié lǐwù, zhēn de, zhā xīnle lǎo tiě.)
This translates to:
"a single man still has to accompany his friend to buy his girlfriend a valentine gift, seriously, 扎心了老铁"