[ros.in] n [ME, modif. of MF resine resin] (13c): a translucent amber-colored to almost black brittle friable resin that is obtained by chemical means from the oleoresin or deadwood of pine trees or from tall oil and used esp. in making varnish, paper size, soap, and soldering flux and in rosining violin bows
[2]rosin vt ros.ined ; ros.in.ing (15c): to rub or treat (as the bow of a violin) with rosin