godan ~ぐ verb / intransitive verb:

• to become brilliant; to become cheerful

adjectival noun:

• upbeat and freehearted; cheerful and magnanimous - four character idiom

godan ~く verb / intransitive verb:

• to float
• to become merry; to be cheerful ➜ 浮かぬ顔
• to become loose; to become unsteady
• to feel out of it; to be cut off (e.g. from those around you); to feel out of place - colloquialism
• to be frivolous; to be uncertain
• to have (time, money, etc.) left over; to be saved (e.g. money)
• to have no basis; to be unreliable

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

noun / adjectival noun:

• innate cheerfulness Antonym: 根暗

noun:

• innately cheerful person; happy-go-lucky person; natural optimist