Zygodactylous literally means to have two toes pointing forward and two pointing backwards. Like a Parrot would have, or several other birds. A Psedo Zygodactylous Gesture means that something with out four opposite facing toes, makes a gesture as though they do.
The image started to appear in art in the medieval and Renaissance periods. A popular image became of the Virgin Mary Breast feeding the baby Jesus while making this hand gesture.
It became much like making the "peace" sign of modern day photos. Nobility and commoners would make this same gesture in portraits.
The trend in art is believed to have been started by "el Greco". Around the year 15781 in Toledo or Madrid, Spain, the painter Doménikos Theotokópoulos, originally from Crete, and known as "El Greco" (the Greek), created a painting that is now called "El caballero de la mano al pecho," "The man with his hand on his chest." The painting is a half-body portrait on a dark background of an unknown bearded Spanish gentleman dressed in basic black with white lace trim at his wrist and neck, adorned with a gold pendant and the gold hilt of a sword high up on his left side, with his right hand on his chest.
There is a single peculiarity about this painting, the gesture of his right hand fingers splayed except for the third and fourth digits which are tight together. These two fingers are painted together in such a way that one would assume that they were physically joined.