Seething means boiling with anger held just under the surface. Seething with rage, sat there seething, a seething crowd.
The containment is the point — a seething person has not said anything yet, which is what makes the word so vivid. The literal sense is boiling, and seethe once meant simply to boil: a sodden thing is one that has been seethed. Of a crowd it means moving in a restless mass. Note sodden, which is its old past participle.
1adjectiveseethingBoiling with anger held under the surface.
“She was seething when she found out he had lied to her again.”
2adjectivemovingMoving in a restless mass.
“The market square was a seething mass of shoppers and street sellers.”
seething is a state of contained anger.
Seethe plus -ing, from Old English seothan, to boil. Sodden is the old past participle of the same verb.
Middle English
sethyng
English, today
seething
From Middle English sethyng; equivalent to seethe + -ing.
Etymology from Wiktionary, CC BY-SA