backscratcher
A long-handled tool for scratching one's own back. The name literally means 'grandchild's hand', from the idea that a grandchild's hand is ideal for scratching an unreachable itch.
See also: 麻姑
孫の手で背中をかく。
I scratch my back with a backscratcher.
孫の手 is the everyday modern word for a backscratcher, while 麻姑 is a rarer, literary term referring to the same object via the immortal's legend.
From 麻姑の手 (Mako no te), referring to the long fingernails of the immortal Mako in Chinese legend, which were said to be ideal for scratching an itch. The kanji 孫 (grandchild) is a later reinterpretation.