A friend of mine once pointed out to me that there is something wrong with a society where pathetic and sad are both terms of abuse. This was back in the nineties and we were schoolboys so anything we disliked was sad and anybody we disliked was a sadcase.
I remembered this yesterday when I happened to hear Pinball Wizard by The Who. The chorus, if you can call it that, goes "That deaf, dumb and blind kid sure plays a mean pinball", meaning that he's very good at pinball. "He makes a mean lasagne", means that the lasagne is good.
It made me wonder what the word mean means in this context. It can't mean miserly and it can't mean lowly so mean must mean malevolent, but in a good way.
This in turn reminded me of when I worked briefly as a researcher for a headhunting firm. They had a database of all the candidates they had ever interviewed. It was divided into four categories: bad, ok, good and killer. Killer was the best. I mentioned this to the CEO and he told me that the company always used the language of the ghetto (which would, technically, be Venetian). There's also a thing much talked of at the moment called a Killer App which, according to Wikipedia, is a "computer program that is so necessary or desirable that it proves the core value of some larger technology". App means computer program. So killer must mean "so necessary or desirable that it proves the core value". Does that seem odd to you?
According to the Online Etymology Dictionary pathetic was first used to mean "so miserable as to be ridiculous" in 1937 and sad began to be "inferior" in 1899. Meanwhile, mean began to mean good in about 1900 and killer first meant good in 1937. So all the semantic shifts I had put together happened within 38 years of each other.
Significantly, I can't think of any equivalent in French. Pathetique still means moving and triste still means unhappy. I might investigate some other languages, but this post is getting too long.
Now, there's a well known aspect of slang, especially children's slang, that words can shift meaning to become their opposites. At my school yoi meant no and noi meant yes. That's why good things used to be wicked and are now sick. There's also a common etymological observation that many words once meant themselves and their opposites. That's why when you withhold something you don't hold it in common and when you withdraw you do it on your own. It's preserved in the two meanings of cleave: to split in two or to cling together. But I don't think that either of those principles can apply here. There is no jocular sense to killer. We simply feel that malice is better than suffering and our words follow.
Necessary and desirable, apparently.