noun

• (dated) A friend; a pal.
  "He looked down upon the girl beside him—a daughter of the desert walking across the face of a dead world with a son of the jungle. He smiled at the thought. He wished that he had had a sister, and that she had been like this girl. What a bully chum she would have been!"
• (dated) A roommate, especially in a college or university.
  "Field had a 'chum,' or room-mate, whose visage was suggestive to the 'Sophs;' it invited experiment; it held out opportunity for their peculiar deviltry."

verb

• (intransitive) To share rooms with someone; to live together.
  "Henry Wotton and John Donne began to be friends when, as boys, they chummed together at Oxford, where Donne had gone at the age of twelve years."
• (transitive) To lodge (somebody) with another person or people.
• (intransitive) To make friends; to socialize.
  "I was not surprised to see somebody sitting aft, on the deck, with his legs dangling over the mud. You see I rather chummed with the few mechanics there were in that station, whom the other pilgrims naturally despised—on account of their imperfect manners, I suppose."
• (transitive, Scotland, informal) To accompany.
  "I'll chum you down to the shops."

noun

• (fishing, chiefly Canada, US) A mixture of (frequently rancid) fish parts and blood, dumped into the water as groundbait to attract predator fish, such as sharks.
  "Near-synonym: shark bait"

verb

• (fishing, ambitransitive) To cast chum into the water to attract fish.
  "He began to chum for sharks, using whale oil and chopped whale meat."