唯一
wéi yī
only; sole
only (唯) + one (一) = the one and only
唯 originally meant 'yes' or 'only' — a mouth (口) answering alone, though 隹 mainly provides the sound. Pair that sole answer with one (一) and you have something utterly singular.
他是我唯一的弟弟。
tā shì wǒ wéi yī de dì di。
He is my only younger brother.
这是解决问题的唯一办法吗？
zhè shì jiě jué wèn tí de wéi yī bàn fǎ ma？
Is this the only way to solve the problem?