What is bud?

Friday I picked up a kid trying to get to Huntington Beach. Not a hitchhiker, he had asked me for bus money which I didn't have. so I was driving him as far as Metro Point (which is really not that far). After about 10 minutes of driving he asked me if I "smoked bud" I was like "uh, what? do I what? no, I've never heard of that"...he's like "weed, do you smoke weed....sorry I asked I was going to offer you a hit, that's all I have to give you". To which I replied "thanks, that's not neccessary, I don't mind driving you". uh, what's bud? He tried to assure me that he didn't smoke it either so I asked "oh, do you just deal it for money" (showing my age in my outdated terminology I'm sure...is it still referred to as dealing?) He replied "no I don't deal it" "but you don't smoke it...or deal it?" "right". Right, perfectly clear kid, you have nothing to hide from me, I'm not the cops or your parents but okay you wanna pretend you just carry it around in that winnie the pooh back pack of yours, fine. I dropped him off and that was that but it got me thinking about the things kids have to go through and what they do to learn and boundaries and raising kids. All this and it turned out the "kid" was 20. I'm 23 so...not that big of a difference. Except I don't know what "bud" is. Can someone explain to me why it's called bud?