"Hit it off" means to get along really well. "They hit it off the second they met." There's an analogy here, but no-one seems to admit it! :) I'm thinking of a successful play, eg. hitting a baseball really well, starting a sports game... (I might be on the wrong track here)

hit it off (with someone): Fig. to quickly become good friends with someone. ex. Look how John hit it off with Mary. ex. Yes, they really hit it off. hit it off (with somebody): to be friendly with each other immediately. ex. She hit it off with Dean and soon the two of them set out on a cross-country adventure. ex. We hit it off beautifully – we liked all the same things, and we liked each other a lot. http://idioms.thefreedictionary.com/hit+it+off :)