Why is Middlesbrough FC's nickname The Boro?

"The first nickname is the Boro, and has been in common usage for decades. The name is a shortened version of Middlesbrough."

http://premierskillsenglish.britishcouncil.org/clubs/middlesbrough-fc

Boro is a shortening of borough which is also shortened to brough.

"The word borough derives from common Proto-Germanic "*burgz", meaning "fort": compare with bury, burgh and brough (England), burgh (Scotland), Burg (Germany), borg (Scandinavia), burcht (Dutch), boarch (West Frisian), and the Germanic borrowing present in neighbouring Indo-european languages such as borgo (Italian), bourg (French), burgo (Spanish and Portuguese), burg (Romanian), purg (Kajkavian) and durg (????) (Hindi) and arg (???) (Persian). The incidence of these words as suffixes to place names (for example, Aldeburgh, Bamburgh, Tilbury, Tilburg, Strasbourg (Strossburi in the local dialect), Luxembourg, Edinburgh, Grundisburgh, Hamburg, Gothenburg) usually indicates that they were once fortified settlements."

https://en.wikipedia.org/wiki/Borough

Incidentally, Middlesbrough is pronounced "MID-?lz-br?"
https://en.wikipedia.org/wiki/Middlesbrough