Zoro hunted people bounties to feed himself... people gave him the name pirate hunter... very simple stuff
Zoro: Ahhh, I'm soooo hungry...
Isn't it odd that you don't actually know anything about sailing?
Luffy: No. I told you I was just drifting around!
Besides, I thought you were supposed to be a bounty hunter prowling the seas for prey.
Zoro: I don't recall ever describing myself as a bounty hunter.
I went out to sea to search for this one guy,
and I couldn't figure out how to get back home.
Zoro: I didn't have anything else to do, so I hunted pirate ships,
and I made a bit of a living from it... That's all.
Luffy: What, so you're lost?
Zoro: Don't say it like that!!