A jackrabbit is a large North American hare — an animal like a rabbit, but bigger, longer-legged, and built for open land — with ears so long they look almost too big for its head. Those ears help it hear danger and let heat out in dry country. When it runs, it can leap far. Though actually, it is not a rabbit, even though rabbit is sitting in the name. It is a hare. The name is a blend of jackass and rabbit: the long ears made people think of a donkey (a jackass), and the shape made them think of a rabbit. You will see the word in American nature writing and in cartoons. As a verb, to jackrabbit can mean to start or speed up all of a sudden, like the animal bursting away — more American, a little informal — but the sense you need first is the animal.