When Matt was asked about the title of the song he said:

I used to be a bike messenger in Chicago for a number of years and one of my best friends who I worked with his name’s Virgil but over the radios when you talk to each other you don’t call each other by name. You have like a call number. His call number was 53 and you know on the radio I’d call him 5-3, and we’d meet up on rainy days or whatever long busy days we’d call each other on the radio and meet up and smoke pot and hang out and whatever. He was kind of my leaning post on rough days. A lot of times he would tell me where he was and I would answer with 5-3-10-4 and go meet him and hang
out.