https://en.wikipedia.org/wiki/68%E2%80%9395%E2%80%9399.7_rule

Basically had to do with standard deviations, for something to be very certain it needs to be beyond 99.7% on accurate equipment.

In the social sciences, a result may be considered statistically significant if its confidence level is of the order of a two-sigma effect (95%), while in particle physics and astrophysics, there is a convention of requiring statistical significance of a five-sigma effect (99.99994% confidence) to qualify as a discovery.

The whole incel thing of a sigma male is supposed to be a male that is in the top percentage of males, thats the whole joke.