'down in the dumps'

Finally he is awake. We are hanging out and he says:

I want to rest a little more so I don't go down in the dumps. What does 'down in the dumps' mean mommy?

I explain.

Yeah I feel like that. Today I feel down in the dumps. My whole body is down in the dumps. I think down in the dumps means when you go down underground and you are in such a mess, that means down in the dumps.

(aaah, he just clarified where he heard this phrase, as I was stumped: Toad (from Frog & Toad) was down in the dumps)