Top definition
It's an acronym for Rainbows, Butterflies and Unicorns. It can be used sarcastically or seriously. Sarcastic use includes when a situation or expectations are unrealistic and one has to put on a happy face regardless.

It can also be use ecstatically to describe an especially happy state of being within one's self about something wonderful.
Sarcastic use:

Sue: The boss gave me this 1500 page report to review and edit by the end of the day. That's in 15 minutes. That dipshit thinks it's all RBUs.

Jenny: At least you gave good face.

Ecstatic use:
Sam: How is it going with that fireman you are seeing?

Michael: It's all RBUs. I want to have his babies. LOL.
by somefun April 07, 2011
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