题外话的英文：
digression; mention in passing

参考例句：

All this is a digression
这都是题外话。

Let me add,in parenthesis,…
让我加一句题外话....

If you'll allow a slight digression,...
请允许我说几句题外话....

The fact that I don't like your fiance is neither here nor there what matters is what you feel.
我不喜欢你那未婚夫，这是题外话－－问题在於你意下如何。

Her dryly humorous asides, such as the appallingly dangerous business of learning to use a sling, are a treat.
书中插入的一些题外话颇具冷幽默风格，比如如何学习使用投石器，读来既为其危险性感到可怖，又觉其乐无穷。

digression是什么意思：
n. 离题

That makes me so digress.
那使我离题了。

There is no need to digress further on the sensuous plane
没有必要更多地扯到灵感层次上去。

By my rambling digression, I perceive myself to be grown old.
这么漫笔杂谈，我自觉有种年老之感。

mention是什么意思：
n. 提及，说起，通报表扬；提名表扬
v. 提到，说起，提名表扬，传令嘉奖

I mention it only as an aside.
我只是顺便提及。

Did the concert get a mention in the paper?
报纸上报道这次音乐会了吗?

He mentioned it of his own accord.
他自愿提到了它。

passing是什么意思：
adj. 短暂的；经过的，路过的；目前的；及格的
n. 经过；流逝；去世；及格
v. pass的ing形式

This coin will not pass.
这钱币不通用。

pass a messenger
传递引绳

The evening passed as evenings mostly pass at Italian theatres
那一声过得很平淡，象意大利戏院里的大多数夜晚一样