잖아(요) is used to show that information should already be known to the other person. It roughly equates to 'as you know', 'you should know that...' or 'I told you that...':
알잖아.
Come on... you know.
이잖아!
You know that it is!
맞잖아요!
You know I'm right!
-- 발렌타인 데이를 즐거워했으면 좋겠어요.
-- I hope you enjoy Valentine's Day.
-- 남자친구 없잖아!
-- You know I don't have a boyfriend!
-- 오늘밤 삼겹살을 먹을까요?
-- Honey, shall we eat barbeque pork tonight?
-- 아니... 고기를 안 먹잖아!
-- No... you know I don't eat meat!
-- 잠깐 -- 오늘은 어머니의 생일이에요?
-- Wait -- it's your mother's birthday today?
-- 이전에 말했잖아!
-- Yes I told you earlier!
-- 오늘은 왜 교회에 가요?
-- Why are you going to church today?
-- 오늘 일요일이잖아요.
-- Come on... it's Sunday today!
-- 왜 아주 많은 선물을 사요?
-- Why are you buying so many presents?
-- 다음 주에 크리스마스이잖아!
-- You know it's Christmas next week!