How would you define "ordinary" cruelty?
I would define "ordinary cruelty" as a sort of secondary effect of tactlessness, thoughtlessness, and selfishness. It arises from lack of empathy. I'd extend the definition to include the sorts of things we say in bursts of anger, when we're hurt and instinctively strike out in an effort to hurt others (the person who hurt us or worse, innocent bystanders). In my view, the term "ordinary cruelty" implies that it is commonplace, something we deal with (and probably deal out) unthinkingly on a daily basis, and it doesn't require physical pain.
Literary examples are myriad. In To Kill a Mockingbird, Francis--Scout's cousin--lashes out at her after Christmas dinner: "Grandma says it's bad enough [Atticus] lets you run wile, but now he's turned out a nigger-lover we'll never be able to walk the streets of Maycomb agin. He's ruinin' the family...." Francis doesn't feel that he fits in with his cousins and is forced to play with them, although he obviously feel superior, and he lashes out to hurt them--and it works. This is ordinary cruelty.
In Of Mice and Men, George says to Lennie, in exasperation, "Whatever we ain't got, that's what you want. God a'mighty, if I was alone I could live so easy. I could go get a job an' work, an no trouble. No mess at all, and when the end of the month come I could take my fifty bucks and go into town and get whatever I want." He has accepted the burden of taking care of Lennie, but the burden gets to be too much sometimes, and he vents his frustration and anger, blaming Lennie--who can't help his condition--for the normal life Lennie robs him of. Such unnecessary and cruel comments hurt Lennie.
In Lord of the Flies, Piggy tells Ralph, "I don't care what they call me, so long as they don't call me what they used to call me at school...They used to call me Piggy." Immediately, of course, Ralph introduces him to the rest of the boys as "Piggy." This is intentional but still ordinary cruelty. Boys of this age tend to antagonize one another, so there's nothing out of the ordinary out of it.
It may be important to remember that ordinary cruelty is more insidious than more pronounced forms of cruelty, like the classic example of Hitler depriving the Jews of rights and persecuting them and eventually destroying as many as he could. Ordinary cruelty is something we come to accept as part of life, and thus something we tend to pass along to others without thinking of the suffering we create and contribute to in our thoughtlessness.