putting on a brave face; pretending to be fine; bravado
Describes outwardly showing energy or cheerfulness while actually feeling down, anxious, or unwell. Often used when someone is trying to hide their true feelings.
He was laughing with forced cheerfulness, but he actually looked very sad.
落ち込んでいるのに、みんなの前では空元気を出してしまう。
Even though I'm feeling down, I end up putting on a brave face in front of everyone.
虚勢 is a more formal or literary term for bluff or bravado, emphasizing a show of strength or confidence that is not backed up. 空元気 is more about emotional pretense in everyday situations.
Standard kanji spelling.
Mixed kanji-kana spelling; common in informal writing.
Katakana-kana spelling; adds emphasis or a casual tone.
Compound of 空 (から, 'empty, hollow') and 元気 (げんき, 'energy, vitality'), literally 'empty energy'.