A British Fist is a new fast spreading term for a half boner or erection. A half boner is when the penis is almost erect, often at full erect length but is still movable or bendable.
The term (British Fist) is associated with a half boner due to both being SOFT YET FIRM.
Damn man i've been sporting a "british fist" almost all day and these sweat pants are not helping.
Dude that chick had such a nice ass, she gave me a "British Fist" thats the fourth time today thats happened.