[He leans forward on his elbows, gravely serious]

You can tell how old a horse is by its teeth. When people still got around by enslaving animals, that was useful to know.

Say Farmer Joe got you a horse and you didn't know how long it had before it was likely to die, so you take a peek under the hood--so to speak. Well that makes you an ingrate.

And that's the meaning of the phrase. Just be grateful for what you're given, and so on.

Me? I'd rather have my cake and eat it, too.