The other day, I was writing a text message in which I said that I was raring to go; and it struck me that rare is one of those odd verbs that you only see as a participle. Nobody ever says "I have rared to go", or "I plan to rare a lot on Thursday". It struck me that maybe you couldn't rare; maybe rare wasn't even a verb at all, as with sidling.
But it turns out that you can rare, and indeed often do. You just spell it slightly differently. Rare is a variant pronunciation of rear. Rear is an Old English word meaning raise (indeed if you go back even further in time rear and raise come from the same root). So you can rear a child, for example, without it being rude, or having anything to do with the rear end (which comes from the French).
Moreover, a horse can rear up on its hind legs. Horses do not do this simply in order to get things down from high shelves, they do it because they are impatient to get somewhere. They are rearing to go, or much more commonly raring to go.
Indeed, the first recorded use of the phrase in the OED has a significant apostrophe. It's from a book called Cabin Fever from 1918
‘Yuh ready?’ Foster's voice hissed in Bud's ear. ‘R'aring to go.
Of course, horses only rear children when they are in a stable relationship.
The first of the Inky Fool's equine kan-kan group was in training