noun
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).
彼は人をこけにするような言い方をした。
He spoke in a way that made a fool of others.
「こけの一念」ということわざがある。
There is a proverb: 'Even a fool has one good idea.'
馬鹿 is the common, everyday word for 'fool' or 'idiot'. こけ is rarer and more literary, often appearing in fixed expressions.
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.