secret pay; under-the-table salary
Refers to compensation paid secretly, often to avoid taxes or regulations, or to conceal the true amount. The term carries a nuance of illegality or impropriety.
It came to light that the company had been paying secret salaries to its executives.
Compound of 闇 (yami, 'darkness; illegal') and 給与 (kyūyo, 'salary; pay'). The exact historical derivation is uncertain, but the word transparently means 'dark salary'.