chum vs cobber

chum and cobber both are nouns.

Word	Noun	Adjective	Verb	Adverb
chum	Yes	No	No	No
cobber	Yes	No	No	No

As nouns, cobber is a hyponym of chum; that is, cobber is a word with a more specific, narrower meaning than chum:

chum (noun)	cobber (noun)
a large Pacific salmon with small spots on its back; an important food fish	Australian term for a pal
bait consisting of chopped fish and fish oils that are dumped overboard to attract fish
a close friend who accompanies his buddies in their activities