Lags Why?

When you read anything in the British press about prisons and inmates, they use the word LAGS where did this word come from as most people don't use it out in the street. This word is used again in  The Mirror paper today 4/1/2017. Can someone please tell me? I don't like it. The word convict or inmate and even prisoner is a better word to use. It does wind me up, when this word LAG is used.