Thorn In The Flesh Meaning

(idiomatic) A persistent difficulty or something very annoying that will not go away.

Example: 1959, Ivar Spector, The Soviet Union and the Muslim World, 1917-1958
  Whereas the growth of new middle and upper classes remains a thorn in the flesh to the Communist Party, the military element is inclined to favor this development, in the belief that those who have something at stake are more likely to fight to defend the system in the event of another conflict.
2001 June 13, Kenya National Assembly Official Record (Hansard) - 13 Jun 2001, page 1106,
  Mr. Deputy Speaker, Sir, the post of the Executive Director of the Kenya Roads Board has been a thorn in the flesh of the Minister.
2002, J. F. Verbruggen, The Art of Warfare in Western Europe, page 323,
  Even individual castles could be a thorn in the flesh of a powerful kingdom.