become a thorn in (one's) side(redirected from become a thorn in their side)
become a thorn in (one's) side
To become a constant or persistent cause of annoyance, frustration, or trouble. My professor was always a passionate environmental activist. Apparently, he's become quite a thorn in the oil companies' sides lately. This lawsuit has really become a thorn in my side. Let's just settle out of court.
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