It is the duty of every lawyer to defend their client to the best of their ability.

defend 是动词，由前缀 de-（表示"向下、完全"）和词根 fend（来自拉丁语 fendere，意为"击打、防御"）组成，字面意思是"完全防护"，引申为"辩护、防御"的意思。在律师语境中特指"为当事人进行法律辩护"。