During debates, the professor always acts as the young colleague's shield-bearer.

shield-bearer 是由 shield（盾牌）和 bearer（持有者）组成的复合名词。shield 象征"保护"，bearer 指"携带或承担某物的人"。 shield-bearer 字面意思是"持盾者"，引申为"保护者"或"捍卫者"，指在争论或冲突中主动保护他人的人。