(verb)
to be agitated and moving rapidly (e.g. to be foaming or churning) due to or as if due to boiling;
e.g. The ocean waters began to seethe as the storm grew move violent.
(verb)
in old usage, to boil something or saturate/soak it in liquid, especially to cook some food by this technique;
e.g. The recipe says to seethe the meat in its own broth.
(verb)
to move about busily and rapidly in a crowd, used to describe the behavior of a crowd of people of things, or to be crowded with people/things moving busily and rapidly;
e.g. The station began to seethe with passengers who had gotten off the train.
(noun)
a state of boiling or bubbling;
e.g. I couldn't see any fish through the seethe of the water.