Social terrorism means when someone you know comes to visit unexpectedly and inconveniently¹, often staying for a long time, and you can't tell them to leave without being rude.

Social terrorism（译为“不速之客”或“社交炸弹”）指有熟人在你不太方便的时候突然到访，而且停留很长时间，你又不能很粗鲁地要求人家离开。

For example:

Yesterday, I was just about to go out, and then the doorbell rang. It was Sally, and she invited herself in and stayed for an hour! It was social terrorism!

昨天，我正要出门的时候，门铃响了。原来是莎莉，她也不客气就进了屋还待了一个小时！简直就是个不速之客！