The most current/common definitions on this webpage seems to originate from the movie "Bachelor
Party" starring Tom Hanks.
In the movie "Bachelor
Party" a minor - but important - role is the character "Nick The Greek" a.k.a. "Nick The D...".
As common in
popular folklore, that a.k.a. caught on. (Mostly between males named Nick).
Nick is actually from the Greek word/name Nicholaos. That said, the movie "Bachelor
Party" had it right - however the movie concentrated on the size. The ancient greeks are not known for their interest in size, but the men in ancient Greece are known for their bi-/homosexual tendencies.