its not 'hey up' it's 'hey what's up' it means 'how are you' but young people use it :))

haha ok but no-one says 'hey up' haha

Ok thank you :) but I've heard that expression several times in TV Series. I think it's like an informal greeting :)

''Hey up'' is a casual greeting between chums. No need to analyse it any further.

haha 'sup' yeah you have to be really cool to use that