這個idiom剛出現時是這樣的，那時的意思與今天

完全不一樣，是警告人們不要在言談中提及devil的，提了devil就來找你麻煩了。但沿革到今天意思已經大變，對此網上有解釋：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