这个idiom刚出现时是这样的，那时的意思与今天

完全不一样，是警告人们不要在言谈中提及devil的，提了devil就来找你麻烦了。但沿革到今天意思已经大变，对此网上有解释：This phrase, in its current usage, is a lighthearted way of referring to someone who has unexpectedly come in when being talked about。 However, prior to the 20th century, it wasn’t a lighthearted one. The full form of this phrase is “Speak of the Devil and he will appear”, and was meant to warn people not to talk about the Devil. It was widely known and used by the mid 1600s. Source: theidioms.com