Uptight in an utterly conceited sort of way.
Someone who 'acts' as if they are completely superior towards/around another person.
Someone who's of the perception that they are 'better' (physically and mentally) than another.
"Me and Jen went to Saks the other day, she's a completely different person over there, actin all sadiddy"
"Yo, if you wanna roll and chill tonight we can do it, but don't try to be actin' all sadiddy around my homies"
"I am not goin' to the club with HIM","Stop bein sadiddy and lez' go."
You call a kid this just to make him mad. He may not actually be sad, but by calling him Sad Kid you may provoke him to end up being a Sad Kid. It's basically taking one of your friends and giving him a nickname he doesn't want. It's like trolling, you just do it to get a reaction.