Named after the Borough of Southwark, the town at the south end of London Bridge.
Southwark was a borough by the 12th century, and as London had no other notable suburbs, it was simply 'the Borough'. Southwark itself means 'southern defensive work': it appears in the 10th century as Suthriganaweorc, 'fort of the men of Surrey', and in the Domesday Book of 1086 as Sudweca.