dock

   to copulate with a female

   The expression was at one time confined to copulation with a virgin, using the imagery of pruning.

   This is a convenient place to note that etymologists do not always agree with each other. Farmer and Henley trace this meaning of dock to the Romany dukker. Partridge, in DSUE, looks to the standard English meaning, to curtail, which, in his judgment, 'is obviously operative'. Grose makes no suggestion as to the etymology but reports 'Docked smack smooth; one who has suffered an doctor I domestic afflictions amputation of his penis for a venereal complaint'. EDD correctly reports that dock means to undress, as in 'mun dock this gound off. OED reminds us that the dock in which a prisoner stands comes from the Dutch word for a rabbit hutch. The New Oxford Dictionary of English adds further to our understanding with the definition 'to attach (a piece of equipment) to another', which is one way of describing the copulatory process. My contribution to the debate is to draw attention to a marine dock, a long, narrow, moist space into which a ship moves and may fit snugly. I am sorry that we shall never know what Alfred Holt, the erudite author of Phrase and Word Origins, thought.