A somewhat old-fashioned or literary word for a foolish person or foolishness. Often used in set phrases like 虚仮にする (to make a fool of) or 虚仮の一念 (a fool's single-mindedness).
He spoke in a way that made a fool of others.
There is a proverb: 'Even a fool has one good idea.'
愚か者 is a formal, literary compound meaning 'foolish person', similar in register to 虚仮 but used as a standalone noun rather than mainly in fixed phrases.
苔 (moss) is pronounced identically to 虚仮 (fool) but is a completely unrelated botanical term.
empty threat; bluff; paper tiger
Usually written in kana; the kanji form 虚仮 is rare.
Katakana spelling sometimes used for emphasis or in informal writing.
Kanji form is uncommon; learners will almost always encounter the kana spelling.
The origin is uncertain. The kanji 虚 (empty) and 仮 (temporary) suggest something insubstantial or false, but the word itself may predate the kanji assignment.