To pay someone less than their work is worth, or less than they should have been given. It works in two rather different situations, and it is worth separating them. The first is about fairness over time: a company underpays its cleaners — the wage is legal, the money arrives, but it is too little for the work. This is the sense you meet in news and in arguments about wages, and it carries clear criticism of the employer. The second is about a specific mistake or shortfall: the office underpaid me last month, they underpaid the invoice by three hundred dollars. Here nothing is being judged unfair — someone simply paid less than the agreed amount, and it can be fixed. English uses the same verb for both, and the context decides. The forms are irregular because 'pay' is: underpay, underpaid, underpaid — never 'underpayed'. It is usually transitive (you underpay SOMEONE, or underpay AN AMOUNT), and it appears very often in the passive: workers are being underpaid, I was underpaid. In fact the passive and the adjective 'underpaid' are far more common than the active verb; you will read 'staff are underpaid' ten times for every 'the company underpays staff'. The related noun is 'underpayment', mostly used for the accounting kind.

少付（薪水／款項）、給的錢過低