to 'rob' is a term originated from northern england which means to 'steal' or 'take something unlawfully'. To be 'On the rob' is an extension of this term, and is used when someone is stealing numerous things in a shop, or is planning to 'rob' a house. The term is probably derived from the word 'robber'...as in 'Bank Robber'
"The gypos are on the rob, you want a DVD player for £20"
"I was just walking along, minding my own, when this seagul swooped down and robbed me chips!....gutted"