careless whiskers

When you have stranded wire and want to insert that into a connector, you should take care that all the tiny wires (strands) go into the connector. Any strands that miss the connector's terminal (and go stray), might end up in the vicinity of the connector, where they might be touchable by a human or animal (electric shock hazard!), or they might touch other parts of the electric circuit, and subsequently cause a short (hazard of electric fire, explosion, or ignition of flammable materials/parts nearby).
Coined sometime in this century by Clive Mitchell (aka BigCliveDotCom) in one of his YouTube videos.

"There's some strands of this wire that didn't make it into the terminal. Careless Whiskers, as I tend to call them" or paraphrased
--BigClive, probably while filming a WAGO connector review video