What is the origin of this phrase? Who first threw who under a bus?
It kinda goes back to the mid-1980s, but really didn’t take off in general untill the 2000s.
It’s used a lot on film crews. When done to me in a joking manner ( which is 99% of the time, because who the hell works with someone who throws you under the bus? ) I say, " Ahhh look at all the transmission fluid I got on me ! Yah rat bastid ! ".
General laughter ensues. Usually penis does not. 
I don’t think I ever heard it much before the 2008 campaign, and almost always in the context of a politician distancing himself from, or criticizing, a supporter for the sake of short-term political expediency.
Way back in the eighties there was a show called McCloud. In one episode an officer finds a cowboy hat which is some sort of clue to a crime.
Officer to his sergeant: “Shall I put it under a microscope, sir?”
Sergeant: “Ya can throw it unda a bus fa all I care!”
I would attribute its gaining currency to Glenn Beck. It was one of his favorite sayings, and the timeline is right. I don’t know that I ever heard it with any frequency until he started using it. ETA: But that doesn’t answer the OP’s question about the first person, which might be a litte more problematic.
More broadly there seems to be a meme that of all the things that could run over and kill you, the most frequently given is a bus.
That’s certainly something that would be said in the UK in the 1950’s and 1960’s, a relatively polite way of telling someone to f**k off.
Usually the bus would also be given a number, “Go throw yourself under a number 9 bus”, yup you couldn’t just pick any old bus, it had to be a specific one, and given the lousy services at the time, that could mean a long wait.
Oddly enough, you wouldn’t tell someone to throw themselves under a train.
As in “could be run over by a bus tomorrow.” Pedestrian vs bus RTAs are actually pretty rare but also generic, it could happen to anyone. The phrase gets used in cases where one wants to reference the possibility of sudden death without mentioning something that is more likely to happen. *You should make a will because you might have a heart attack/crash your stupid car/get cancer/overdose from all those drugs you take * is something I’m not even comfortable typing without a specific person in mind.
What does the expression mean, though? The way I’ve always heard it used, it seems to mean to attempt to absolve yourself of blame or responsibility by attributing the undesired consequence to somebody else, like the phrase to “pass the buck” used to be used in the past.
The image I always got is that of two associated people standing alongside a bus when one suddenly throws the other under it from the side where no one can see what is happening. The outcome would be one dead body in what appears to have been an accident. The perpetrator walks away with a “tsk, tsk” on his lips and a “good riddance” on his mind.
this, i also assume the OP meant passing blame onto someone else. other people seem to be using the phrase differently.
If some says “Billy threw me under the bus”, it means that Billy revealed something I said or did that puts me in a bad light or assigns (deserved) blame to me.
It’s not always deserved blame. If something goes wrong in a large agency/company, someone may be picked as a sacrifice to take all of the blame even if the problem was only obliquely concerned with them.
Urban dictionary as a few different definitions for this phrase. I’ve always took “thrown under a bus” to mean being made into a scapegoat, i.e. “That bastard Charles really threw me under a bus when he told everyone that the failed product launch was entirely my idea.”
I don’t know if it ties in, but westerns often had John Wayne or whoever telling the villain “There’s a stage leaving at five o’clock. Be on it.” I recall hearing people in the mid-1970s (when pop culture was starting to become “cool”) changing the tag to “Be under it,” in humorous variant. “Under the bus” could have evolved from that.
Interesting; I always took the meaning to be “deserved”, but not public knowledge.
I first heard it (to great excess) while watching Project Runway. It’s a great phrase for reality TV. When your team screws up, the first thing you need to do is find a scapegoat and throw them under the bus. Or if you don’t, you brag about how you didn’t throw them under the bus. Or if you were the scapegoat, you complain about being thrown under the bus. Blah.
I don’t watch a lot of reality TV so I don’t know how prevalent it is/was on other shows. It’s also not the best expression in that regard; I always thought they should say something about tripping people running away from grizzly bears.
This is the OP’s sense. Some others seem to be of a different intent.
ETA: To attribute blame or responsibility to someone who had previously been an ally, which is different from passing the buck.
I usually interpret it as joining in an attack (or at least, conspicuously not defending) on a person who you were previously allied to. Not exactly the same thing as scapegoating, since there’s no requirement for a scapegoat to be allied to the scapegoater, but similar.