Thorn in one’s side

What is Thorn in one’s side?

Thorn in one’s side definition and meaning on Dictionary terms:

a sharp excrescence on a plant, especially a sharp-pointed aborted branch; spine; prickle.
any of various thorny shrubs or trees, especially the hawthorns belonging to the genus Crataegus, of the rose family.
the wood of any of these trees.
a runic character (