Jobs a good'un?

why when you finish a job,do you say: jobs a good'un?

I would imagine that it is simply an informal way of saying that in your opinion the 'job (that has just been finished)is a good one'.

DFG

That phrase is commonplace in the North of England, which is where I imagine taccy62 heard it. I don't know the origin. It just seems like colloquial speech. "Good 'un" is an everyday expression meaning "well done" or expressing approval. I would guess it is just a straightforward extension of that.