Sigma is a noun that first appeared in English in the 1600s.
Traditional definition: Name for the eighteenth letter of Classical and Modern Greek: corresponding to the S in Latin
First citation (OED): Church of England clergyman and author Edward Topsell in 1608.
Evolution: Variations appeared in science and maths, but it remained specialised and relatively obscure. Then popularised as internet slang in the 2020s
How it is used in current English (slang):
self-assured that individual who stands apart from the crowd. Usually seen as a model of status, success, and attractiveness. Sigma can also mean “extremely good,” but is also widely used as a nonsense term and meme online. Merriam Webster