Meaning:

“回忆”literally means “memory or reminiscence,” and “杀”is “to kill.” Originally, this phrase first appeared in dramas and anime. Before a character in a drama or anime gets killed or is ready to kill his or her boss, a flashback scene will appear. Now it’s widely used among netizens to refer to someone who gets deeply touched when something reminds him or her of the good old days. For example, when a man who has been working for many years hears a song that he once listened to in high school, he may be filled with strong emotions and feelings since the song may remind him of his time in high school.

Example:

A: 昨晚李若彤在微博发了她跟古天乐的合影，你看到了吗？

Zuówǎn lǐruòtóng zài wēibó fāle tā gēn gǔtiānlè de héyǐng, nǐ kàndào le ma?

Did you know that Carman Lee posted a photo of her and Louis Koo on her Weibo last night?

B: 看到了，真没想到95版电视剧《神雕侠侣》的两位主演还能同框。真是一波回忆杀！

Kàndào le, zhēn méi xiǎngdào 95 bǎn diànshìjù “shén diāo xiá lǚ” de liǎngwèi zhǔyǎn háinéng tóngkuāng. zhēnshì yī bō huíyìshā.

Yes I did. I never thought the two leading roles of the TV show “Condor Heroes 95” would take a photo together one day. It reminded them of how sweet the two characters were in the TV show 24 years ago.