What's "D" word?

Gib: I know what the “D” word is.
Me: You do? (I give Mike “the look” across the table. I hope he can read my mind because here’s what I’m thinking: “Way to go, Mike. I told you to quit your $#%! swearing. The kids hear you when you %@#$! swear at the dishwasher! And at all the other inanimate objects in our house. This is all your fault. It’s certainly not mine, since I stopped my @$#&! swearing the first time Gibson parroted it right back to me at the tender age of 2. You got that, buddy? Are you reading my mind? You’d better be getting all of this…..” )
Gib: Yeah. But I can’t say it because it’s bad.
Me: It is?
Gib: Yeah. But I know what it is.
Me: Well what is it?
Gib: I can’t say it.
Me: Really?
Gib: Yeah. Cause it’s bad.
Me: I want you to tell me. Why don’t you come and tell me in my ear.
Gib: Are you sure?
Me: Yeah. It’s Ok. You can tell me in my ear.
Gib: OK.

He climbs down out of his chair, comes to my side of the table, stands on his tip-toes, leans into my ear and whispers “The D-Word means “Shut-up”.