They're asking what kind of underwear you're wearing. Without more context, it is kind of a strange question to ask someone.

Boxers are men's underpants that are loose-fitting and have short legs. They look somewhat like a boxer's trunks. Briefs are snug, more tight-fitting, with no "legs" - hence the name. Boxers are considered more conservative. A "briefs" day might be considered more daring (though perhaps slight).

在哪看到的？:-P就是説 你今天要穿boxers還是briefs？

i think if they say is it a boxer or brief day to you? it refers to a financial aspect the way i see it why. boxer shorts is kind of loose and comfort for me.while breif is kind of tight.boxer day refers that you have enought money for the day and brief day is your littele bit short in terms of money... Cowboy..

trunks - which are something in between boxers and briefs, a skin tight version of boxers