Sigma is a noun that first appears in English in the Seventeenth Century.
Traditional definition: Name for the eighteenth letter of Classical and Modern Greek: corresponding to the S in Latin
Usage:The OED cites the first usage as by Church of England clergyman and author Edward Topsell in 1608. Variations appeared science and Maths but it remained specialised and relatively obscure until being popularised as internet slang
Contemporary definition:
self-assured, dogged individualism is considered a model of status, success, and attractiveness. It can also mean “extremely good,” and is also widely used as a nonsense term and meme online. Merriam Webster