colloquial language - Why does 微妙 become "sucky" in slang usage?

Technically the word 微妙 means

as per WWWJDIC:

(adj-na,n) (1) delicate; subtle; sensitive; (2) difficult; delicate (situation); complicated; (3) doubtful; questionable; dicey;

However, last time I stayed in Tokyo ( about a year ago) most of my Japanese friends (about 25-30 years old) were using 微妙 to mean "kinda sucky" or "boring in an annoying way". I believe I remember a sentence like this:

海はどうだった？ まあちょっと微妙。

How was the beach? Eh... it kinda sucked.

I also heard it used as an adverb with this meaning but I can't remember an example. When I asked my friends about it, they said they don't really use that word for positive things, though none of the numerous examples I saw on ALC had this slangy type of meaning. I guess the meaning of subtle mixed with dicey and then became "so either-or that it just sucks"? If anyone can explain the subtleties of this slang usage, I've been wondering for a while.

Answer

微妙{びみょう} has been going through an evolution during the time I've been in Japan. There was a time, maybe about a decade ago, when it seemed to be one of those popular words that people would over use. Similar to how there was a fad for a time, mostly with younger people, to add 超{ちょう}("ultra~") to almost everything for emphasis.

Being that slang is, by nature, fluid in its evolution and open to interpretation, I think you'll see a lot of variety in daily use, and the dictionaries, like ALC, might not be keeping up.

I don't think it means "kind of sucky," however. To me, "kind of sucky" is a clear judgment that something is "sucky", with the only question being to what degree, answered by appending "kind of." (Though I'll admit we might just be disagreeing on the definition of "sucky", which is itself slang, so I hope you can go along with the spirit of what I'm trying to convey if not the literal terms.)

微妙{びみょう} retains its lack of certainty even when it tends to the negative. So I would take your 「まあちょっと微妙{びみょう}」 to be just a little more vague. Something like "Meh, the beach didn't really do it for me." In other words, it wasn't that it was clearly bad, maybe there were some things that were okay, and maybe other people liked it, but me, I was nonplussed.

Used in its adjectival form, on example that I hear often is 「微妙{びみょう}に違{ちが}う」, which means something like "[it's] kind of off [somehow]". I think in this phrase, it's the 違{ちが}う that's doing more of the heavy lifting of conveying the negative than 微妙{びみょう}.

Just for comparison, similar terms are 中途半端{ちゅうとはんぱ}, which can go from "half done" to "half assed", or 今一{いまいち} which is literally "not quite" but can mean "almost but not actually good".

Hope that helps.