Someone said something funny in the Kagerou Days novel I read, but I need to explain a word to explain the joke. Here goes!
冴える (さえる） (ru-verb) crisp, clear; awake, alert
The kanji 冴 is the ice radical (the two dots on the left) plus “fang” (牙), and the word 冴える originally meant “become bitterly cold.” Bring a jacket, it’s ice fangs out there today!
Nowadays, 冴える isn’t as much of a weather word. It means “crisp and clear” when you talk about sights, sounds, etc (I guess that’s how things look on a cold winter day?)
When you’re talking about someone’s mind, 冴える means alert or smart. Like your thoughts are coming through all cool and crisp and clear. 目が冴える (eyes being clear) means that you’re wide awake and can’t go to sleep.
Meanwhile, 冴えない means that something is …kind of meh. Dissatisfying. Boring. Dull.
Now you’re ready for minor Kagerou Days novel spoilers!