gakubori; background elements in traditional Japanese tattooing
Refers to the background designs (waves, clouds, rocks, etc.) in traditional Japanese tattoos (和彫り), as opposed to the main motif (主図).
In traditional Japanese tattooing, waves and clouds are carved as gakubori around the main motif.
Compound of 額 (gaku, 'frame' or 'picture') and 彫り (hori, 'carving'), referring to the carved background that frames the main design.