Green’s Dictionary of Slang

sad n.2

also sado
[SE sad]

a general term of abuse for someone perceived as dull and unfashionable.

OnLine Dict. of Playground Sl. 🌐 sad, saddo, sad caseadj, n., Commonly used everywhere to describe people who don’t fit in, don’t have any style, or wear the right clothes to be part of any faction. These people are the nerds and geeks of the world.
[UK]J. Meades Empty Wigs (t/s) 326: But it was always other people, poor sadoes, who got romantically involved with that electric jezebel the Hoover Dustette when cleaning the stairs.
[UK]J. Meades Empty Wigs (t/s) 821: You know those disturbed sads craving attention claiming they perpetrated foul kiddy murders when they were a hundred miles away at the time.