ros ‧ in /ˈrɒzən, ˈrɒzɪn $ ˈrɑː-/ BrE AmE noun [uncountable]
[ Date: 1200-1300 ; Language: Old French ; Origin: resine ; ⇨ ↑ resin ]
a solid slightly sticky substance that you rub on the ↑ bow of a ↑ violin etc, to help it move smoothly on the strings
—rosin verb [transitive]