• 不安ふあん — anxiety / unease
• 不便ふべん — inconvenience
• 不満ふまん — dissatisfaction
• 不可能ふかのう — impossible
• 不思議ふしぎ — mysterious / strange

Derived from a shape originally related to plants or buds, 不 came to represent negation or contradiction. It’s one of the most frequently used prefixes to reverse or negate the meaning of a word in both classical and modern Japanese.