And I always thought that an equine cock horse was simply one which lifted its front feet high when trotting, so bringing its leg horisontal, a Human cock horse was just the thigh for a baby to be jounced on - and that in days past the request to 'cock up' meant to put your foot up against a wall or on a suport of some kind so that your mate could climb up on your thigh in order, for instance, to reach the apples or other fruit dangling temptingly overhead.

My mother's mother had a large collection of old books, from visiting auction houses to furnish her large old house between the wars, and most of them predated the motor car, so the hours spent reading there did rather antiquate my vocabulary.

I have also heard the term cock horse for the extra horse used to help get vehicles up hills, but as Hackneys are high trotting and also used to pull carriages I though that was the connection.

My Grandad used the term 'cock(ed) up' for 'prick(ed) up' - as in what a dog does with its ears.

I suspect that it is simply an alteration in the naughtiness of words.