Two distinct uses: (1) family — your maternal cousin or paternal aunt's son (i.e. cousin through the female line, parallel to
表哥/
表弟). (2) regional nickname — "
江西老表" ("Jiangxi mate") is the long-standing affectionate label for people from Jiangxi province, used the way "Buckeye" works for Ohio. Origin stories vary; one popular legend traces it to migrations during the Ming dynasty.
