I'd like to know in depth detail, what is the meaning of this expression?
How come there is only a few people saying it? Old and not died is a good thing in my mind.
老不死 == 老而不死 being old, but not die; It implies some old guy should die as soon as possible, rather than stay alive, wasting food and hurting others... etc. It's used to curse some old people whom you hate and consider to be useless to others.
It doesn't mean Chinese think old is bad, instead Chinese people have a tradition to support old people.. Whereas, if there's someone who did nothing good when he was young, accomplished nothing when he was an adult, stays alive when old, and keeps hurting people around, his life then is considered useless and even harmful to others. You then can call him 老不死, to curse him to die early.
Also, there are some people who hate old people and think they are useless all over the word. For this kind of people, they might just use the word 老不死 to refer to any old people, in a derogatory way.
The original of the word was from Confucius. 子曰: 幼而不孙弟, 長而無述焉, 老而不死, 是為賊.
I just want to supplement Fivesheep's answer with some examples.
The closest equivalent I can find for
老不死 in English is "old fart". This is a very disrespectful way of addressing a senior person whom you despise.
Why haven't that old fart move out of the house?
Although the term
老不死 is usually used in a derogatory manner, it doesn't preclude some women from using it on their husband. It really depends on the tone and the level of relationship between the addressee and the addresser.
Why the hell did you come back home so late?
The angry wife is swearing (not cursing) at her husband for coming home late, perhaps suspecting him of doing something up to no good.
This is quite similar in usage to
死鬼老公 which some women may use on their husband when in a foul mood. In this case, it doesn't mean that the wife want her husband dead and become a ghost.
In China, if you call anyone as 老不死, you are not praising him as "old and not died". In fact, you are asking him "why don't you go die?". So we think it's so impolite to call anyone 老不死, especially call elderly. Besides, there is a saying "老而不死是为贼“, it means that if someone is so old but never go die, then there must be something weird with him. Thus, nobody in China would apperciate you if being called 老不死.