fer ‧ rule /ˈferuːl, ˈferəl $ ˈferəl/ BrE AmE noun [countable] technical
[ Date: 1600-1700 ; Origin: verrel (15-19 centuries) (influenced by Latin ferrum 'iron' ), from Latin viriola 'small band worn around the arm' ]
a piece of metal or rubber put on the end of a stick to make it stronger