I had forgotten about that!

My first thoughts was Cory Doctorow's Down and Out in the Magic Kingdom. They call is "subvocal" or "subvocalizing" in the novel as well. Not saying he came up with the term though, just pointing it out.

You can read the whole thing for free on his website: https://craphound.com/down/Cory_Doctorow_-_Down_and_Out_in_t...