He dislikes others interfering in his personal affairs and always keeps his thoughts in his backyard.

backyard 的字面意思是"后院"，指住宅后面的私人户外空间。在比喻用法中，backyard 常用来表示"私人领域"或"内心世界"，强调不愿对外公开的私人空间或想法。这里用 backyard 来比喻"内心深处"，形象地表达了他不愿向他人透露想法的状态。