means Covered in thorns, or — more often these days — describing a problem so prickly and difficult that handling it leaves a mark.

from From Old English 'thorn,' the same word that named the spiky plant and the old runic letter þ (which spelled the 'th' sound). The '-y' simply means 'full of' — so 'thorny' is literally 'full of thorns.' The leap from bramble to bothersome problem is an old one: by the time of Early Modern English, anything bristling with hidden snags could be called thorny, the metaphor practically pricking itself into being.