A codename for a absolute gigachad. Except they aren't, because gigachads don't exist, and neither should sigmas.
A sigma is a 14 year tiktoker who thinks having more muscle than human body is a nessicary part of being masculine. Like, sir do you need some body to go with that muscle? So essentially to be sigma, just give up your physical appearance, iq, chances of getting a girl, you being straight, your respect, and don't forget to give up your chances of having friends.
"That guy's a sigma!"
"True sigma"
"I hate sigmas"