A flashy and sometimes un-tasteful piece of clothing , usually a really bad Hawaiian short-sleeve button-up.
Could also apply to someone who doesn't seem to look good in modern fashion or has retired to Florida.
Tom- Jeez Mat did you see Jake wearing that ugly shirt?
Mat- Yea that's such a Brett Shirt, he should hire a personal clothing coach.
When you buy Dunkin Donuts and you sit in a gutter and you have a nice time
Man I need to make some space for my annual gutter-time session