"Cowsy"
I grew up with two older sisters, who didn't much get on at the time. As such, they used to call each other "cow" quite a lot. As an infant, I latched onto it, realising it was faintly taboo or insulting. I assume I added the "sy" bit myself.
Apparently I'd scream it at my Mum when I was pissed off with her taking me around the shops. Needless to say, I was quite an awkward child.