What is 550 First Avenue?
A term used to indicate that someone is unbalanced and/or needs to seek medical attention for a mental disorder.
550 First Avenue is the location of the New York University Department of Psychiatry - one of the largest psychiatry faculties of any department in the United States. Whether the subject is neuropsychiatry or psychoanalysis, psychopharmacology or behavorial therapy, positron emission tomography or epidemiology, child psychiatry or geriatric psychiatry, there is an expert in this field.
Dave: "Did you see John last night - acting all weird? The guy is losing it!"
Bob: "Yep, we'll be sending his Christmas card to 550 First Avenue soon enough."