Verb (1-dan, intransitive)
1. to be clear (of a sight, sound, colour, etc.); to be bright; to be vivid; to be crisp
冬の夜空に月が冴えている。The moon shines clear in the winter night sky.
2. to be clear-headed; to be alert; to be on the ball; to be wide awake
今日の自分は冴えている。I am sharp today.
3. to look upbeat; to be perky; to be cheerful
often as 冴えない
なんだか冴えない顔をしてるわね？You look kind of glum, don't you?
4. to master (a skill); to excel at; to cleanly execute
基本があるから、技が冴えるんだよ。It is because there are fundamentals that your techniques shine.
5. to be satisfying
often as 冴えない
6. to become frigid; to become intensely cold
original meaning; now somewhat old-fashioned