1847, originally of a type of reed organ, variant of melodion, from German Melopdoin, from Melodie, from Old French melodie (see melody). As "a music hall" by 1840.
mid-14c., junke "old cable or rope," cut in bits and used for caulking, etc., a nautical word of uncertain origin, perhaps from Old French junc "rush, reed," also used figuratively as a type of something of little value, from Latin iuncus "rush, reed" (but OED finds "no evidence of connexion").
It was extended to "old refuse from boats and ships" (1660s), then to "old or discarded articles of any kind" (1884), usually with a suggestion of reusability. Meaning "salt meat used on long voyages" is from 1762. Meaning "narcotic drug" is from 1925. Junk food is from 1971; junk art is from 1961; junk mail first attested 1954; junk bond from 1979.
Modern sense of "air-tight vessel of tinned iron" is from 1867. Slang meaning "toilet" is c. 1900, said to be a shortening of piss-can; meaning "buttocks" is from c. 1910, perhaps extended from this.
c. 1400, "artillery piece, mounted gun for throwing projectiles by force of gunpowder," from Anglo-French canon (mid-14c.), Old French canon (14c.), from Italian cannone "large tube, barrel," augmentative of Latin canna "reed, tube" (see cane (n.)). The double -n- spelling to differentiate it from canon is from c. 1800. Cannon fodder (1847) translates German kanonenfutter (compare Shakespeare's food for powder in "I Hen. IV").