The Spanish have a perfectly respectable word mondongo for the tripes, the stomach linings of cows or oxen that are served as food. Many people adore tripe, especially served with onions, but others find it repulsive to varying degrees. Hence our slang use of tripe for worthless stuff or rubbish.
The English borrowed the Spanish word in the seventeenth century, at first with the same sense, but then hacked it about a bit to fit English mouths, so creating mundungus, and applied it figuratively to any offal or refuse.
Later, it was used in particular for a foul-smelling form of cheap tobacco. In his Journal of A Voyage to Lisbon, published in 1755, Henry Fielding wrote: “It was in truth no other than a tobacco of the mundungus species”. It has largely gone out of use, except when an author is attempting to reinforce an historical period, as Patrick O’Brian does in The Ionian Mission: “If you have finished, Stephen, pray smoke away. I am sure you bought some of your best mundungus in Mahon”.
It’s gained a higher recognition factor in recent years because of the character Mundungus Fletcher who appears in several of the Harry Potter books by J K Rowling. Mr Fletcher is a bad’un, “a smelly sneak thief”, a liar and a cheat, so his name is apposite. Here he’s up to his tricks in Harry Potter and the Goblet of Fire: “Mundungus Fletcher’s put in a claim for a twelve-bedroomed tent with en-suite Jacuzzi, but I’ve got his number. I know for a fact he was sleeping under a cloak propped on sticks.”