Keep it up

[Rihanna]

If said in a cheerful, praising tone, it means Youre doing great, continue the good work!

If said in a warning tone, it is a threat that means Stop it right now, or reap the consequences!

[Jason Yan]

Sarcastic phrase used when one really wants to say something like shove off, fuck off, eat shit, kick rocks, go to hell or simply stop.

[Rihanna]

It means to keep your dick hard during sex.