get on like a house afire

Of two or more people, to enjoy one another's company very much from the start and become good friends at once. (Used when people are meeting for the first time.) I can't wait for you to meet Mark, I just know the two of you will get on like a house afire! I was worried about my university friends meeting my new girlfriend, but everyone got on like a house afire. A: "I thought you and Heather would get on like a house afire." B: "Really? Ugh, she's so annoying."