A green cap is a reference to a semi-erect penis. The phrase "green cap" is linked to the color of the lids that are found on Semi-skimmed milk bottles.
*Sergio* Don't you think Alex is looking a bit happy with Sophie sitting on his lap?
*Josh* Yeah mate, kid's got a green cap fo'sho!
the people you can trust when running or riding in the naked bike ride at university of vermont. since bramley took away the police and what not, the green caps are here to save you if you get hurt or assaulted. also wearing green hats...
help, i've fallen and i can't get up! someone get a green cap.