An 18th birthday party which is a while after their actual birthday. A lateenth can be anywhere from a couple of weeks late up to 6 months late (after 6 months it becomes an early 19th)
Person 1 "hey are you going to Brady's 18th?"
Person 2 "he turned 18 a month ago"
Person 1 "I know it's a lateenth"
Person 2 "he turned 18 a month ago"
Person 1 "I know it's a lateenth"
by Gravy Train 97 November 12, 2015