O.E.
seoþan "to boil" (class II strong verb; past tense
seaþ, pp.
soden), from P.Gmc.
*seuthanan (cf. O.N.
sjoða, O.Fris.
siatha, Du.
zieden, O.H.G.
siodan, Ger.
sieden "to seethe"), from PIE base
*seut- "to seethe, boil." Driven out of its literal meaning by
boil (v.); it survives largely in metaphoric extensions. Fig. use, of persons or populations, "to be in a state of inward agitation" is recorded from 1588 (implied in
seething). It had wider fig. uses in O.E., e.g. "to try by fire, to afflict with cares." Now conjugated weak, and pp.
sodden (q.v.) no longer felt as connected.
[sod] 
noun, verb, sod⋅ded, sod⋅ding.