thorn in (someone's) side

(redirected from a thorn in their side)

thorn in (someone's) side

A constant or persistent cause of annoyance, frustration, or trouble. My professor was an environmental activist when he was younger, and, apparently, he was quite a thorn in the oil companies' sides at the time. My little brother is such a thorn in my side—he always wants to do stuff with me, especially when I'm trying to hang out with my friends.
See also: side, thorn