It's strange how a phrase will sit in your brain from childhood, for over twenty years for no good reason. There's a Muppets sketch with "Doctor Bob" where they're operating on a patient of some sort - it's probably a muppet sort of patient. There's a delightfully awful pun in there.

Nurse: Doctor Bob - the patient is sinking.
Doctor: I wonder what he is sinking about.

Very poor. Later on, I seem to think there's another gag.

Nurse: Doctor. We've lost the patient.
Doctor: No. He's just here, under the sheet.

Totally harmless. Very sweet.

I guess I'm thinking of it at the moment as my mood sinks down from the highs of a damned good trip away from... erm... all of... this.