I voted for "homeless adventurers that kill orcs and take their stuff".
However, I feel the term is broader than that and that the gap between "homeless adventure that kill orcs and take their stuff" and "bandit that kills peasants" is pretty darn small.
Murderhobo refers to a style of play where the PC's have no real attachment to anything, no real place in the world, and no motivation beyond leveling up and getting loot. Since leveling up and getting loot is usually most easily done by killing things and taking their stuff, the PC's are generally actually motivated to do this and any other motivation for doing that is just an excuse. Thus, it really doesn't matter that the orcs are evil or more particularly whether they've actually done anything worthy of death or vengeance. The point is that they have stuff and are walking XP sacks. The PC's aren't in fact motivated by any sort of noble sentiments and in fact, in the long run they'll evolve whatever mode of play is the most efficient way to gain XP and wealth safely.
At that point, "powerful adventurers that bully commoners" and "killing orcs and taking their stuff" tend to overlap. What tends to happen is the players realize that the quest giver that they previously relied on to point them to the stuff to kill and loot is sending them on increasingly difficult and lethal errands, and that equally good loot and XP is to be had more easily and safely looting the town, robbing the bank, or killing the quest giver. Thus, for example, a party playing 'Village of Homlett' or 'Keep on the Borderlands' might realize all the best loot is in the village or the keep. If you have real murderhobos, and it turns out looting the village or the keep is actually easier than digging equivalent loot out of the dungeon, then the PC's will find themselves an excuse for looting the human village as readily as they looted the orc village.