The klingon term for slang is {mu'mey ghoQ}, which translates to "fresh words".

But this confuses me. Can't a language (klingon in our case), possess slang words which are pretty old ?

A generation of teenagers grows up and stops using slang. But then the next generation comes, adopts these words until the next generation comes and so on..

So, why is it necessary for a slang word to be "fresh" ?

We don't know. It could be a fossilized expression. The origin of the English word slang itself appears to be uncertain: one theory says it originally meant "special vocabulary of tramps and thieves." It no longer means that, and yet we still use the word.

-- 
SuStel
http://trimboli.name