(verb)
to be friendly with someone, to spend time with them and become friends with them (usually used as "chum around");
e.g. They started to chum around after class and eventually became close friends.
(noun)
(American) a substance that is thrown into the water as bait when fishing, especially something like chopped fish;
e.g. They had a bucket of chum with which to attract the sharks.
(verb)
to throw bait into the water in order to attract fish, or to attract fish by throwing bait into the water;
e.g. They usually chum the larger fish from the deep water.