1. A description used for someone who has done better than you, for instance beaten you in a test or something similar. It is generally used when the other person is showing off and rubbing their glory into your face. It is slightly sarcastic and yet praising at the same time.
2. Also used for a description of something that is utterly amazing or cool, that is usually new and has never been seen before.
1. Amy: Yay! I got 25/25 in my Latin Test Jess!
Jess: Flash pants.
2. Hattie: Have a look at my new pencil case, Callum.
Callum: Wow, flash pants!