The word 不愉快 is formed by prefixing the negation character 不 (meaning 'not') to the existing word 愉快 (meaning 'happy'). 愉快 combines 愉 (conveying pleasure) and 快 (in this context, contributing a sense of pleasantness), so the resulting compound 不愉快 directly conveys the meaning 'not happy' or 'unhappy'.