He felt honour-bound to fulfill his promise to his friend despite the difficulties.

honour 的意思是"荣誉、道义"，指高尚的道德准则或行为规范。 bound 是动词 bind 的过去分词，bind 的意思是"捆绑、约束"，这里表示"受约束的"。 honour-bound 是复合形容词，字面意思是"被荣誉所约束的"，引申为"出于道义必须做的"或"在道德上有义务的"。