speak of the devil 是在18世纪初开始流行的。当时有一种迷信的说法，也就是：speak of the devil and he's sure to appear。如果你说到魔鬼，魔鬼就一定会现身。对，后来就简化成：speak of the devil，这和我们中文的习语“说曹操，曹操就到”有异曲同工之妙。

【例句】
Shall we go? Oh, speak of the devil, here he is now.
噢，他来了，真是说到曹操，曹操就到。

We were just talking about Frank when he came in the door. Speak of the devil and he appears.
我们正在谈论弗兰克的时候弗兰克就进了门。真是说到曹操，曹操就到。