Alice here:

The boy's reaction tells us a lot about Old Cotter. 

My mother worries when I drive the car, and I think that's dumb.  Of course I'm not going to do anything stupid with the car; mom's got to let me grow and try things.  That's how I feel.  In sort of the same way, the boy here has little patience with Old Cotter who would keep him from information.  But we will see that every syllable Old Cotter has either said or implied is  true! 

And look at this.  Look at the word the boy calls Old Cotter, "imbecile." 

It comes from a Latin word bacillum, prefaced by the negation in (in bacillium) which literally means without a rod, or without a staff . 

If that doesn't make an enormous light bulb flash, click here. 

Now, if the light bulb has flashed, then here's the rest of  the joke: it's not even a big staff Old Cotter's lacking.  According to the boy; he doesn't even have a little staff (bacillum means little rod or staff).  (Nobody in the story gets the joke. It's a little subliminal joke from Joyce to us.)

If none of this sounds convincing to you, click here. Maybe click there anyway.