1. (某些植物、树木上的)刺，棘刺
    Thorns are the sharp points on some plants and trees, for example on a rose bush.

    e.g. Roses will always have thorns but with care they can be avoided.
           玫瑰花上都有刺，不过只要小心就碰不到。

2. 荆棘;带刺的树;带刺小灌木
    A thorn or a thorn bush or a thorn tree is a bush or tree which has a lot of thorns on it.

    e.g. ...the shade of a thorn bush.
           一丛荆棘下的阴凉
    e.g. ...the thorn and bramble thickets.
           荆棘密布的灌木丛

3. 肉中刺;长期困扰的问题
    If you describe someone or something as a thorn in your side or a thorn in your flesh, you mean that they are a continuous problem to you or annoy you.

thorn在线翻译

    e.g. She's a real thorn in his side...
           她真叫他头疼。
    e.g. The Party was a thorn in the flesh of his coalition.
           该党是他所在联盟的眼中钉、肉中刺。