答案解析："Old fellow" in the English sentence "My heart's broken, old fellow" is used as a term of endearment or a friendly address, similar to "old friend" or "pal." It's a colloquial way of addressing someone, often used between men, to express familiarity or camaraderie. The phrase "My heart's broken" suggests a feeling of sadness or disappointment, and the speaker is sharing this with the person being addressed.