1
general::
Phrase(s): hit it off (with someone)
Fig. to quickly become good friends with someone. • Look how John hit it off with Mary. • Yes, they really hit it off.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Get along well together, as in I was so glad that our parents hit it off. In the 17th century this phrase was put simply as hit it, the adverb off being added only in the mid-1800s.
American Heritage Idioms