a rounded piece of wood or plastic used in darning to support the fabric around the hole
darning egg in American English
noun
a smooth, egg-shaped piece of wood, ivory, marble, jade, or the like, for holding under a hole or tear to serve as a backing while darning
Word origin
[1895–1900]This word is first recorded in the period 1895–1900. Other words that entered Englishat around the same time include: Marxism, apothecaries' measure, calling card, freewheel, hit-and-run