Dock is the term that references the mail sex organ. It's a combination of two slang terms for the penis. Those terms are "dock" and "cock". Putting them together you get a great term to use for the wang.
From the hit movie "Magnolia". The information starring Frank T.J. Mackie says the following while promoting his "how to get girls" series of tapes called "Seduce & Destory". He says.... "...if you buy my tapes you are going to have all those women's panties moist and they won't believe how badly they will want to wet you dock!"