Yobbo or yob is a slang

Slang

Slang is the use of informal words and expressions that are not considered standard in the speaker's language or dialect but are considered more acceptable when used socially. Slang is often to be found in areas of the lexicon that refer to things considered taboo...

 term for an uncouth or thuggish working-class

Working class

Working class is a term used in the social sciences and in ordinary conversation to describe those employed in lower tier jobs , often extending to those in unemployment or otherwise possessing below-average incomes...

 person. The word derives from a back slang

Back slang

Back slang is an English coded language in which the written word is spoken phonemically backwards. It is thought to have originated in Victorian England, being used mainly by market sellers, such as butchers and greengrocers, to have private conversations behind their customers' backs and pass off...

 reading of the word "boy" (boy or boyo reversed becomes yob or — slightly modified — yobbo).
Dr. C. T. Onions, an editor of the Oxford English Dictionary

Oxford English Dictionary

The Oxford English Dictionary , published by the Oxford University Press, is the self-styled premier dictionary of the English language. Two fully bound print editions of the OED have been published under its current name, in 1928 and 1989. The first edition was published in twelve volumes , and...

, seems not to have noticed the word before he published the XYZ section of the dictionary in 1921, but later supplement notes the use of 'yob' as meaning 'boy' in the working-class youth context, from 1859.