Text-killer is the final text, usually a one word reply, such as "yeah" and "haha ok", used to designate the end of the conversation. Unless you have a desperate texter on the other end. 

短信终结语就是最后一条短信，通常是一个词的答复语，比如“是啊”或“哈哈好的”，表明对话结束。除非另外一方想没完没了地聊下去，否则对方看到短信终结语一般都会停止发短信。

Example:

Rachael doesn't know how to take the text-killer and stop talking!

瑞秋看不懂别人的短信终结语，发起短信来没完没了！