keep it up

If said in a cheerful, praising tone, it means "You're 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!"

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.

It means to keep your dick hard during sex.

Keep pace with, take or give the time to someone to follow while working on the same project or being in the same team.
Continue/manage the proper continuance of fulfilling any financial obligations.

an arby's saying when their food keeps blowing up

An alternative to 'keeping up with the joneses', although hardly used in the common English lexicon. Other examples include--

A) In expression, used to describe surprise and happiness for the end of the week - Friday, a day in which school is typically over, or for those in work. It can be used also for Thursday itself, for people waiting for the week to be over.

B) A title for a popular webcomic of the same name, about two mousethieves in London with a knack for stealing. The title stems from the fact that the main character, Patrick, was known as the 'Thursday Thief', for his crimes coincidentally happening on that day. Hence, people are trying to 'keep up with Thursday.'

C) A term used to describe the end result of an orgasm during sexual intercourse; the success of a wonderful night in bed with your mate and/or lover. Commonly used in the United Kingdom, though included in the United States in some instances.

to strive beyond one's means to keep up socially and financially with others in one's social circle or neighborhood (from the name of a comic strip by Arthur R. Momand)