ichidan verb / intransitive verb:

• to be clear (of a sight, sound, colour, etc.); to be bright; to be vivid; to be crisp
• to be clear-headed; to be alert; to be on the ball; to be wide awake
• to look upbeat; to be perky; to be cheerful - often as 冴えない
• to master (a skill); to excel at; to cleanly execute
• to be satisfying - often as 冴えない ➜ 冴えない
• to become frigid; to become intensely cold - original meaning; now somewhat old-fashioned