An almalgamation of the phrases 'safe' and 'nice one' this common used phrase has it's roots in Cambridge surburbia, but is found in all areas of the UK
Tali: "I did like a million pieces of work today!"
Tom: "Safe one babes"
This is a confused attempt to combine the word 'safe' with the phrase 'nice one'. Usually blurted out in pressure situations in the company of intimidating individuals.
Intimidating individual- "There you go bruv"
You- "errr... safe one"