What is the Baader-Meinhof phenomenon? What are the origins of the expression?

It is the illusion that when you hear something specific once, you are more likely to notice it again. It is considered to be a combination of being sensitive to the term and confirmation bias. The same might happen if you buy a new car and suddenly notice a lot more cars like your new one on the road.

The origin of the expression "Baader-Meinhof" was first proposed by Terry Mullen who mentioned it as a German terrorist group. After mentioning the group Terry kept noticing the name popping up.

https://en.wikipedia.org/wiki/Frequency_illusion