1.proper nounA design flaw in computer systems that represented years as two digits (e.g. 84 for 1984), which could cause date calculations to fail if they involved years later than 1999.
"Mr Johnson told Tory supporters at a private dinner that gloomy economic forecasts were "mumbo jumbo" and the Northern Irish border row had been blown out of proportion, as fears over disruption to supplies of food and medicines were “pure millennium bug stuff”."