Business casual.
I went in about the same as Timothy Schlang and all was fine.
I went in shorts and tshirt and sandles they said nothing but nice casual is your best
I would suggest smart casual
Business casual will be your best bet.
Can't go wrong with business casual attire at Fat Canary.
It's pretty laid-back. I would feel uncomfortable rocking athleisure, but I'm not sure if they'll turn you away...
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