rosin, n. — Green’s Dictionary of Slang

rosin n.

[? SE rosin, a sticky material that is smeared on a violin string or bow, to facilitate its playing; fiddlers were assumed to be drinkers; or Irish raisín, a snack]

1. liquor.

1734

cited in Partridge DSUE (1984).

2. beer or other drink given to the musicians who entertain at a dance or party; thus rosin, to supply the musicians with drink.

1821		‘An Amateur’ Real Life in London I 408: After a little more rosin, without which, he said, he could not pitch the key-note, he sung the following.
1864, 1867, 1870		Hotten Sl. Dict.

3. (also rosin-the-bow, rozin) a fiddler, a violinist.

1852		Yankee Notions Oct. 289: [caption to illus of dancing couple and violinist] Keep it up, Betsy! Down with them are hoofs of your’n! Go it, old Rawzin Scraper!
1864, 1867, 1870		Hotten Sl. Dict. 215: ROSIN-THE-BOW, a fiddler.
1897		S. Watson Life’s Look Out 32: Who-ay, Cully, here’s Hoppy with the rozin .