To give a high five to someone for a suitably smutty suggestion, especially in the company of one's parents or colleagues.
To count as a Dagenham Five, the fingers of the slapping hand must be in the shape of a Dagenham Handshake.
Pete: I'd like to hot carl that ho my man. Where the seram wrap at y'all?
Shane: Oh no you didn't. Dagenham five! <hand slap>