It's incredibly obvious, actually, for the exact lack of effort he's putting into the explanation.

At the threat of being hexed, Childermass can only raise his eyebrows, looking much more interested in this little attempt to scare him than actually, you know, scared. Small angry nerd, is that really something that can run in a family? Miko seemed to have avoided such a fate, somehow.

"Maybe I'm mad at him over the rude things about cats he said," he carries on with that lame-ass story regardless, then adds quite casually, "Though I would be careful about hexing the person who's here to open the door for you. It would be a shame if you ruined everyone's plans."