chum¹ / chəm/ inf., dated • n. a close friend. ∎  a form of address expressing familiarity or friendliness: it's your own fault, chum.• v. (chummed, chum·ming) [intr.] be friendly to or form a friendship with someone: they started chumming around in high school.DERIVATIVES: chum·mi·ly / ˈchəməlē/ adv.chum·mi·ness n.chum·my adj.chum² • n. chopped fish, fish fluids, and other material thrown overboard as angling bait. ∎  refuse from fish, esp. that remaining after expressing oil.• v. [intr.] use chum as bait when fishing.chum³ (also chum salmon) • n. (pl. same or chums ) a large North Pacific salmon (Oncorhyncus keta) that is commercially important as a food fish.

chum one who shares rooms with another, (hence) intimate associate. XVII. prob. short for chamber-fellow (XVI), orig. a word of Oxford univ. sl., corr. to the Cambridge crony.
Hence chum vb. share rooms. XVIII.