What does a crown of thorns mean?

Any affliction that causes great suffering. As in, when Jesus Christ had the crown of thorns place on his head.

It is also a trailing or climbing spiny shrub (Euphorbia milii) native to Madagascar and cultivated as a houseplant, having showy flower clusters with usually red, petallike bracts.

usually used to describe someone bit*chy. The meaning has some religious aspects, and im pretty sure its an ancient torture device lol, but modern day slang wise, its pretty much used to describe somebody who thinks they are the most awesome person around but do not realize their actions have reactions. hence thorns on a crown.