Do a Rishi

to do Rishi (or a Rishis): Common British slang for unofficial work.
unofficial work often making use of company equipment/ time/ premises but solely for personal gain, unrelated to the interests of the company and/or tax department. With notable exceptions, doing Rishis is a fairly accepted part of British working life and, depending on the workplace, can even be done with managemental consent.

I'm notcoming to the pub this lunchtime, I'm staying at work to do a Rishi for a mate of mine.