A twat cap are caps which were originally worn by rappers and gangsters but have now been worn by hipsters, the caps are usually have an american basketball team logo or a baseball team logo, they usually make the wearer look like a twat
"Hey look at that Hipster with his twat cap"
"I really want a twat cap but its too mainstreamnow"
The tag used on photos taken on June 14th, where you put a hat on someone who you think is a twat, then take a picture of them, while telling them of the tradition.