from spasm, or spastic
1) One who has a proclivity for wild, random outburst of activity (to be a spaz).
2) An unusual display of random clumsiness (to throw a spaz)
3) An uncommon display of exceedingly random behaviour over a brief period of time (to spaz out).
2) He threw a spaz and dropped everything.
3) When he heard that his hamster had died he, like, totally spazed out.
to spaz, spazzing.
"My leg/arm/head/otherbodypart's spazzing" - which is when aforementioned body part twitches erratically.
"She's totally spazzing." - when somebody is going mad ^^
Also can apply to people who are nervous, jumpy, hyper and needs to take medication to calm down. Uusally a spazz will blurt out unneccasary things to get attention.