Most likely if it's "406" on tshirt, it means a telephone area code, so person is from Montana.

Less likely, and somewhat dated, it's from Zombieland, a 2009 American post-apocalyptic horror comedy film. The main actor lives in an apartment complex, and has a crush on a woman in apartment 406. She turns into a zombie, and his first encounter with a zombie. He eventually killed her with the lid the tank of his toilet.

Very unlikely it's "406" the error code returned by an Internet server meaning Not Acceptable.
Why is that guy wearing a tshir with "406?"

He's probably a "lamb licker" from Montana
by Kingdragonfly August 08, 2018
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n. A term derived from the movie "Zombieland".

It is used to refer to an attractive female. Usually, it must be a neighbor of yours for it to even make sense since you are referencing the movie.
Guy 1: "I have a new neighbor that just moved in."
Guy 2: "And you're telling me this because?"
Guy 1: "Dude, she's a 406!"
Guy 2: "Have you met her? Can I meet her? Let's kick it at your place right now!"
by Certified Smartass November 24, 2010
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A girl who hypothetically has 406 or less followers, but understands people who have 406, 000 followers. She has approximately 15-20 Instagram posts that include photos from homecoming, prom, graduation, one with their besties, one with their pet(s), etc. A 406 typically is very fashionable and creative. 406s typically major in subjects like: business, finance, interior design, modeling, etc (or have jobs in these fields). They also love spending time with their family. 5ft or above is preferred.
I won’t date her unless she’s a 406.

They type of girl I want to marry is a 406.
by @matthewrdang June 19, 2019
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A discrete way to refer to a hot female, quoted from the movie Zombieland about Amber Heard.
Dude, look at the 406 at that table! I wish she would try eating me!!!
by j4883rwocky November 13, 2010
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A Nazi submarine that suffered failure on launch twice.
U-569 makes the contact and leads them.
U-94 scores a kill in the dark.
U-124 sinking 4 in 2 approaches.
U-406 suffers failure on launch agaiiin!
by anonymous6496782 August 24, 2019
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A U-Boat that suffers failures on (torpedo)launches. twice.
569 makes the contact and lead them
U-94 scores a kill in the dark
124 sinking four in two approaches
U-406 suffers failure on launch AGAIN!
by Never gonna give you up! February 14, 2021
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