The narrator is a woman, half drunk and walking along the edge of parapet in the dark, and suddenly a stranger calls her behind her.
'I don't think you should stand there.'
I half turn, and catch a flash of a small pale face on the fire escape, dark eyes wide open. In shock, my foot slips on the parapet, my weight suddenly the wrong side of the drop. My heart lurches, a split second before my body follows. And then, like a nightmare, I am weightless, in the abyss of the night air, my legs flailing above my head as I hear the shriek that may be my own-
-After You by Jojo Moyes
Here, I don't understand the meaning of the part marked in bold especially because drop has many definitions and also I don't get what "wrong side" means in this context.
Also, what's the verb omitted in the bolded part? is it slip?