The term missy appears to be a Chinese Pidgin English term from English miss + the Cantonese phonotactic suffix -ee/-y[.]

What I suffered with that rein for four long months in my lady's carriage, it would be hard to describe, but I am quite sure that, had it lasted much longer, either my health or my temper would have given way. Before that, I never knew what it was to foam at the mouth, but now the action of the sharp bit on my tongue and jaw, and the constrained position of my head and throat, always caused me to froth at the mouth more or less.

They were top-heavy, blunt-nosed ambulances, painted gray and built like moving-vans. […] Three others were up in the mountains at dressing stations.

That be 'cause she don't care for ye singsters, an' no dancin' girl'll stay where the profits be so lean.