This word is a fairly rare thing, but a much rarer thing – so rare as to seem to be erring – is to see it without to right after it.
And raring to what?
I bet you said go. Yup, far and away, raring to go is where you see this word. Occasionally it will show up with something else – raring to try, raring to fight. And sometimes it will have a bit more before it. Come on, now, tell me what two words would come before raring to go.
Yup, ready and.
Raring makes me think of drag racers, at their start line, the engines revving – you can hear them in the sound of “raring” – and when the flag drops the tires smoke and the nose of the car may even rear up a bit.
Hm, rear up. How about a horse that’s rearing up because it’s so eager? Rearing to go?
That would be the source of this. Raring is a variant (originally southern US) of rearing, as in rearing up on hind legs. Raring has always had the broader sense of ‘excited, eager’ or ‘angry, wild’, though, not just the specific one of ‘rearing up’. You might sometime have seen raring and tearing, but not so likely recently. It has also been used as an intensifier, as in a raring good time. And why not? It has that aggressive, desirous /r/ sound, helped by the /e/ vowel. It’s not quite the gripping /gr/ phonaestheme, but it still has some Tony the Tiger in it.
Raring may seem to have the vigor and juice of rare steak, but it’s not related to that rare. The rare of steak actually comes from an old word rear that refers to undercooked things, such as underdone eggs. The origin of that word is uncertain; it could be related to rear as in ‘behind’ – though probably not – but the rear as in ‘behind’ is not related to the rear that rare comes from (the ‘behind’ one seems to come from arrear).
Quite the set of rears, isn’t it? A bit of a pain in the behind. The rear that became raring is related to the word raise and used to have most of the same senses. Now it shows up in just a few: rearing up on hind legs, child rearing, things rearing their (often ugly) heads. The rest of them have been put on the rear ranges, or otherwise rearranged. But even as that form faded from use, and its modified version rare slipped away, raring managed to stay in a fixed form.
Try this, it might be fun: Ask someone why we say raring to go. See what kind of explanation they come up with.