Middle English sethen, from Old English seoþan "to boil, be heated to the boiling point, prepare (food) by boiling," also figurative, "be troubled in mind, brood" (class II strong verb; past tense seaþ, past participle soden), from Proto-Germanic *seuthan (source also of Old Norse sjoða, Old Frisian siatha, Dutch zieden, Old High German siodan, German sieden "to seethe"), from PIE root *seut- "to seethe, boil."
Driven out of its literal meaning by boil (v.); it survives largely in metaphoric extensions. Of a liquid, "to rise, surge, or foam" without reference to heat, from 1530s. Figurative use, of persons or populations, "to be in a state of inward agitation" is recorded from 1580s (implied in seething). It had transitive figurative uses in Old English, such as "to try by fire, to afflict with cares, be tossed about as in turbulent water." Now conjugated as a weak verb, its old past participle sodden (q.v.) is no longer felt as connected.