my favourite definition is,
1# stay up in life. dont let life knock u down u know. smile n stay shine, even if your broke or just got robbed, life is still fine.
2# from the graffitiworld, stay up is like get yo shit out there, get up and dont get x-ed or buffed.
3# simply a good-bye
1# from emcee identitys song stay up:
cuz feelin blue wont help ya, only make u depressed
so stay up i say and dont live life stressed
2# man u gotta get out there, u gotta get up n stay up
3# iight homie, hit ya up tomorrow, stay up