The adjective derisory means so absurd that it makes you laugh, like your friend's derisory offer to buy your valuable baseball card collection for a dollar.
You can use this adjective in two ways. First, it means "ridiculously inadequate." Things in this category include derisory pay for doing a difficult job and the derisory punishment your sister gets for stealing your Halloween candy. The second meaning is "mocking or expressing contempt," basically a synonym for derisive. The Latin root of both words is deridere, "to ridicule."