Those who know the least are the most likely to be overconfident

I believe the answer is Dunning-Kruger effect

Dunnig-Kruger effect happens when Someone with low ability suffer from illusory superiority,

This make them actually believe that they're actually better than other people even though they do not possess any actual achievement/proof to backed their perception.

Actually its not always the case, but its really yes, for few people, who are very over confident with least or not complete or no clear knowledge, but still they show off as if they know everything, which is not good, which manytimes will lead those people into shame in front of other people who knows everything as much clearly and hence this negative attitude should be changed to positive always in life.