I’m taking the liberty of posting a second WOTD, but unlike the first, it’s a verb. It came up in a conversation I was having yesterday, and I realised I hadn’t used it in a very long time, which is apropos for me, because it takes me a long time before I get to the point where I’m seething.
The word comes from the old English seothan, meaning to make or keep boiling. I find it to be very graphic. I immediately get a picture in my mind of a boiling pot with steam bursting out the sides, or Yosemite Sam, getting angry and having steam blow out his ears. It works pretty well with simmering as well, sort of the precursor to seething I guess.

His voice got darker, humour turned dimmer,
One look said he was starting to simmer,
He checked his pulse and also his breathing,
No doubt now that he finally was seething!