"How was that restaurant?"
"…微妙。"
You think: interesting, nuanced, complicated. The restaurant is finished. Everyone at the table has already updated their plans. You are the only person still hopeful.
Bimyou is a real word with a real meaning — subtle, fine, delicate, as in a subtle difference. That respectable dictionary version still exists, and it is the reason the slang version works so beautifully.
Because in conversation it has become the standard soft rejection. Something is bad, or disappointing, or not going to happen, and rather than say that — which would require somebody to lose face — you describe the situation as "delicate". Everyone understands. Nobody has been rude. The verdict is unanimous and unstated.
You will hear it about films, dates, restaurants, job offers, weather forecasts and, painfully, your own Japanese. It is the same social machinery as the famous 難しいですね ("that's difficult, isn't it"), which means it is impossible, and 考えておきます ("I'll think about it"), which means it is over.
Once you can hear bimyou properly, a hidden layer of Japan switches on. You stop mistaking politeness for agreement, and you start noticing how much information is being transmitted at extremely low volume.
Foreigners complain that Japanese people never say no. They say no constantly — you are just failing the listening test. Bimyou, 難しい, ちょっと…, a slow intake of breath through the teeth: these are all full-strength refusals. The message is not hidden. It is simply sent at a frequency you have not tuned into yet.