sig block = S = sig virus

sig quote /sig kwoht/ n.

[Usenet] A maxim, quote, proverb, joke, or slogan embedded in one's sig block and intended to convey something of one's philosophical stance, pet peeves, or sense of humor. "Calm down, it's only ones and zeroes."

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.