And what is a "Tzakpar"? When I was working on the painting, my son Jonathan came to me – he was an important "critic" when he was a child – and I couldn't tell him what it was, or perhaps I was trying to be a wise guy. I told him this is a "Tzakpar". I did feel that this word is not in the dictionary. He asked me: What is a "Tzakpar" and I answered him: This is a "Tzakpar". I guess the answer satisfied him. Perhaps he let go, in any case, he stopped asking. It was enough for me. 
(From an interview with Yona Fisher, Kav 6)