(there hasn’t been a unified etymology thread in like 4 years so I’ll start this new topic)
This is one of those things I just got used to after joining the local 青年会 a couple years ago, but now I’m wondering. Where does this alternate meaning for 青 come from?
In old Japanese, there wasn’t really a distinction between blue and green. In this case, it means something more like green and it indicates the same thing as “green” in English when we’re talking about someone inexperienced. The connection in both cases is to plants, being fresh and green while they are young. In the English example, youth is associated with inexperience.
I think green meant here is as in “not ripe” veggies or fruits that are often green before they ripe.
In Germany, there is that saying that someone is still “green behind their ears” to indicate inexperience. Sometimes funny, how same concepts/similar idioms exists in different culture/languages.